drivers: mdio_nxp_enet: Don't disable IRQ

No real need to be enabling and disabling IRQs, this logic has been
reported to be causing spurious interrupts and strange behavior, we can
just enable the interrupt and switch to interrupt based logic one time
and keep the interrupt enabled at that point.

Also, fix a W1C bug where |= was used instead of = to clear a flag.

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