Change if statements in autoip.c to fix compiler warnings

[kamendov-maxim]

Got warnings about unused code when tried to build example app with clang-14 so I moved options checks to preprocessor if statements

[yarrick]

What warnings did you get?

[kamendov-maxim]

Unused lines of code if LWIP_DHCP_AUTOIP_COOP is not 0

/lwip/src/core/ipv4/autoip.c:308:21: error: code will never be executed [-Werror,-Wunreachable-code]
autoip->state = AUTOIP_STATE_CHECKING;