net: fec: Fix RMON registers on imx6

commit 38ae92dc "fec: Add support for reading
RMON registers" causes the imx6Q to crash.

This fixes it by only enabling the RMON registers, the
registers are already cleared by the MAC being reset.

Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>