net: fec: fix build error in no MXC platform

build error cause by
Commit ff43da86
("NET: FEC: dynamtic check DMA desc buff type")

drivers/net/ethernet/freescale/fec.c: In function ‘fec_enet_get_nextdesc’:
drivers/net/ethernet/freescale/fec.c:215:18: error: invalid use of undefined type ‘struct bufdesc_ex’
drivers/net/ethernet/freescale/fec.c: In function ‘fec_enet_get_prevdesc’:
drivers/net/ethernet/freescale/fec.c:224:18: error: invalid use of undefined type ‘struct bufdesc_ex’
drivers/net/ethernet/freescale/fec.c: In function ‘fec_enet_start_xmit’:
drivers/net/ethernet/freescale/fec.c:286:37: error: arithmetic on pointer to an incomplete type
drivers/net/ethernet/freescale/fec.c:287:13: error: arithmetic on pointer to an incomplete type
drivers/net/ethernet/freescale/fec.c:324:7: error: dereferencing pointer to incomplete type etc....

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>