mt7622: linksys-e8450: enable using mt7531 switch irq

Turns out the MT7531 switch IRQ line is connected to GPIO#53 just like
on the BPi-R64, so this seems to be part of the reference design and
will probably apply to most MT7622+MT7531 boards.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>