net,fec: Shorten device name as done for other drivers

After discussion on the ML it is suggested to drop unrequired
and not useful characters from the device name.
This patch changes the name for the fec_mxc driver from
"FEC_MXC" to "FEC".

Signed-off-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>