staging: mt7621-eth: Lock is never unlocked.

mtk_phy_link_adjust takes a spinlock and disables
interrupts, but never unlocks.
This can leave interrupts disabled on one CPU and
various things stop working.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>