drivers/net/calxedaxgmac.c: Fix compile warning

drivers/net/calxedaxgmac.c: Fix compile warning

Fix this:
calxedaxgmac.c: In function 'xgmac_rx':
calxedaxgmac.c:479:2: 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 unsigned char *'
calxedaxgmac.c: In function 'calxedaxgmac_initialize':
calxedaxgmac.c:547:12: warning: assignment from incompatible pointer type

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