net/fec: preserve MII/RMII setting in fec_stop()

Additionally to setting the ETHER_EN bit in FEC_ECNTRL the MII/RMII
setting in FEC_R_CNTRL needs to be preserved to keep the MII interface
functional.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>