4xx_enet.c: fix GCC 4.6 build warnings

Fix:
4xx_enet.c: In function 'enet_rcv':
4xx_enet.c:1772:21: warning: variable 'ef_ptr' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>