net: fec_mxc: adjust prototype of fec_get_miibus() for DM_ETH

commit 306dd7da ("net: fec_mxc: fix PHY initialization bug with CONFIG_DM_ETH")
has broken the build of the fec_mxc driver with CONFIG_DM_ETH
enabled because it changed the parameters passed to *fec_get_miibus()
without changing the functions prototype.

This patch fixes up the prototype of fec_get_miibus() for the DM_ETH case.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>