driver: mdio: mdio_nxp_enet: Implement c45 read/write

This change refactors the mdio_read / mdio_write functions into an
mdio_wransfer, and utilizes it to implement
mdio_{read,write,read_c45,write_c45}.

Heavily inspired by the implementation in drivers/mdio/mdio_sam.c

Signed-off-by: John Barbero Unenge <git@lsrkttn.com>