net: fec_mxc: Fix MDC for xMII

The MDC signal is available on all xMII (i.e. 'not 7-wire') interfaces, so
mii_speed has to be set for all these interfaces, and not only for MII.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>