arch/powerpc/cpu/mpc8220/fec.c: Fix compile warning

arch/powerpc/cpu/mpc8220/fec.c: Fix compile warning

Fix this:
fec.c: In function 'mpc8220_fec_recv':
fec.c:791:9: warning: passing argument 1 of 'NetReceive' discards qualifiers from pointer target type
include/net.h:438:13: note: expected 'uchar *' but argument is of type 'volatile uchar *'
fec.c: In function 'mpc8220_fec_initialize':
fec.c:839:12: warning: assignment from incompatible pointer type

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