drivers/net/mpc5xxx_fec.c: Fix build warning

Fix:
mpc5xxx_fec.c: In function 'mpc5xxx_fec_initialize':
mpc5xxx_fec.c:920: warning: assignment from incompatible pointer type

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Anatolij Gustschin <agust@denx.de>