net/fec: restore interrupt mask after software-reset in fec_stop()

After the change from mdio polling to irq, it became necessary to
restore the interrupt mask after resetting the chip in fec_stop().
Otherwise, with all irqs disabled, no communication with the PHY will be
possible after e.g. un-/replugging the cable and the device gets
stalled.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>