net: phy: meson-gxl: fix interrupt support

If an interrupt is already pending when the interrupt is enabled on the
GXL phy, no IRQ will ever be triggered.

The fix is simply to make sure pending IRQs are cleared before setting
up the irq mask.

Fixes: cf127ff2 ("net: phy: meson-gxl: add interrupt support")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>