drivers: slip: Use always the initialized network interface

No need to always look through all the network interfaces
when receiving packet as we support only one interface and
it is always the same one.

Change-Id: Ic0bfabc8409160352081cac1a6c3dbbfa35fe390
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>