enc28j60: rx fiber's stack size is increased

The rx fiber call contiki IP stack functions to register
received frames. This interaction requires more stack memory
in the rx fiber stack to avoid memory corruption.

Change-Id: I106339e7dd1ca9282426adfb54590e34297755b3
Signed-off-by: Juan Manuel Cruz Alcaraz <juan.m.cruz.alcaraz@intel.com>