net: context: Use signed value for interface index

As the value 0 is a valid network interface index, we cannot use
unsigned value for interface index as that would not allow to
distinguish an invalid value. So make interface index a signed
8-bit value which is ok as we do not expect to have more than 127
network interfaces in the system.

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