net: fec_mxc: Drop CONFIG_FEC_XCV_TYPE

With all boards now using DM_ETH we determine the value for
CONFIG_FEC_XCV_TYPE at run time, except in the case of the default
fall-back.  Set the fallback directly now.

Cc: Fabio Estevam <festevam@gmail.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>