[ecnconfig] Handle Multi ASIC backend port names when extracting port I/F

[smaheshm]

- What I did
On Multi ASIC platforms ports names for backend ports have the following convention: 'Ethernet-BPxy'

Port ID extraction was failing for these backend port names.

- How I did it

Fix the port ID extraction for backend port names.

- How to verify it

Manual:

before:
admin@str-sonic-acs-2:~$ sudo ip netns exec asic0 ecnconfig -q 3
Exception caught:  invalid literal for int() with base 10: '-BP48'

After:
admin@str-sonic-acs-2:~$ sudo ip netns exec asic0 ecnconfig -q 3
ECN status:
queue 3: on

- Previous command output (if the output of a command-line utility has changed)

- New command output (if the output of a command-line utility has changed)

[smaheshm]

retest this please