drivers: mdio_nxp_enet: Fix busy wait

Fix the busy wait in the MDIO driver that was causing timing
problems in systems with real time requirements performing tasks
more frequently than about a millisecond.

Restructure the code to be less redundant and change the busy wait
kconfig to microseconds instead of millliseconds. Also actually signal
to the mdio driver that it can use the interrupt instead of busy
waiting, this seems to have been forgotten.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>