net: if: Check return value of k_poll() when sending data

Check the return value of k_poll() as instructed by Coverity.
If debugging is enabled we print error if this happens, otherwise
this error is ignored. In our case the return value should always
be 0.

Coverity-CID: 173652

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>