net: fec_mxc: simplify fec_get_miibus

No need to provide two prototype for this function.
Use ulong for the first parameter, then this function
could be shared for DM/non DM case.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>