drivers/net/enc28j60.c: Fix compile warning

Fix this:
enc28j60.c: In function 'enc28j60_initialize':
enc28j60.c:967:12: warning: assignment from incompatible pointer type

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>