MPC8xx: fix "Remove volatile from net API" aftermath

Commit db288a96 "net: Remove volatile from net API" caused a number of
build warnings:
fec.c: In function 'fec_initialize':
fec.c:183:13: warning: assignment from incompatible pointer type [enabled by default]
fec.c: In function 'fec_recv':
fec.c:284:5: warning: passing argument 1 of 'NetReceive' discards 'volatile' qualifier from pointer target type [enabled by default]
/home/wd/git/u-boot/work/include/net.h:433:13: note: expected 'uchar *' but argument is of type 'volatile uchar *'
scc.c: In function 'scc_initialize':
scc.c:85:14: warning: assignment from incompatible pointer type [enabled by default]

Fix these.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Joe Hershberger <joe.hershberger@ni.com>