adc.rs: fix channel -> smp register mismatch

[Lotterleben]

While read()ing a Pin mapped to channel 10 we got an (unexpected to us) panic:

 ERROR panicked at 'internal error: entered unreachable code', /Users/lottesteenbrink/.cargo/git/checkouts/stm32f3xx-hal-bb439b50181a2760/6997410/src/adc.rs:610:1
└─ panic_probe::print_defmt::print @ /Users/lottesteenbrink/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-probe-0.2.1/src/lib.rs:95
────────────────────────────────────────────────────────────────────────────────
(HOST) WARN  no RAM region appears to contain the stack; cannot determine if this was a stack overflow
stack backtrace:
   0: HardFaultTrampoline
      <exception entry>
   1: lib::inline::__udf
        at ./asm/inline.rs:172:5
   2: __udf
        at ./asm/lib.rs:49:17
   3: cortex_m::asm::udf
        at /Users/lottesteenbrink/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-0.7.3/src/asm.rs:43:5
   4: rust_begin_unwind
        at /Users/lottesteenbrink/.cargo/registry/src/github.com-1ecc6299db9ec823/panic-probe-0.2.1/src/lib.rs:76:9
   5: core::panicking::panic_fmt
        at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:92:14
   6: core::panicking::panic
        at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:50:5
   7: stm32f3xx_hal::adc::Adc<stm32f3::stm32f303::ADC3>::set_chan_smps
        at /Users/lottesteenbrink/.cargo/git/checkouts/stm32f3xx-hal-bb439b50181a2760/6997410/src/adc.rs:509:30
   8: stm32f3xx_hal::adc::Adc<stm32f3::stm32f303::ADC3>::convert_one
        at /Users/lottesteenbrink/.cargo/git/checkouts/stm32f3xx-hal-bb439b50181a2760/6997410/src/adc.rs:471:21
   9: <stm32f3xx_hal::adc::Adc<stm32f3::stm32f303::ADC3> as embedded_hal::adc::OneShot<stm32f3::stm32f303::ADC3,Word,Pin>>::read
        at /Users/lottesteenbrink/.cargo/git/checkouts/stm32f3xx-hal-bb439b50181a2760/6997410/src/adc.rs:538:35
        
 [... backtrace dives into our own code from here]

Which from my understanding is caused by a channel/SMP Register mismatch in set_chan_smps(). This PR fixes that.

[Sh3Rm4n]

Thanks for fixing this oversight! 👍

I guess, this would be a candidate for backporting to v0.8, to release it earlier, as breaking changes are already on the master. I'll add it to my to-do list.