net: dsa: mv88e6xxx: Release lock while requesting IRQ

There is no need to hold the register lock while requesting the GPIO
interrupt. By not holding it we can also avoid a false positive
lockdep splat.

Reported-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>