net/fec: fix MMFR_OP type in fec_enet_mdio_write

FEC_MMFR_OP_WRITE should be used than FEC_MMFR_OP_READ in
a mdio write operation.

It's probably a typo introduced by commit:

e6b043d5


netdev/fec.c: add phylib supporting to enable carrier detection (v2)

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>