imx/gpio: Use handle_level_irq

According to Russell King handle_edge_irq is only useful for "edge-based
inputs where the controller does not remember transitions with the input
masked."

So using handle_edge_irq unconditionally for both edge and level irqs is
wrong.  Testing showed that the controller does remember transitions
while the interrupt is masked.  So use handle_level_irq unconditionally.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>