net: fec_mxc: Fix setting of RCR for xMII

net: fec_mxc: Fix setting of RCR for xMII

At least on i.MX25, the RMII mode did not work, which is fixed by this patch.

The MII_MODE bit of the FEC RCR register means xMII, i.e. 'not 7-wire', so set
it accordingly.

According to the xMII and 7-wire (aka GPSI) standards, full duplex should be
available on xMII, but not on 7-wire, so set FCE accordingly. The FEC may
support full duplex for 7-wire too, but the reference manual does not say that,
so avoid an invalid assumption. Actually, the choice between half and full
duplex also depends on the endpoint/switch/repeater configuration, so a config
option could be added for that, but there has been no need for it so far.

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