ARM: OMAP: Clear level-triggered GPIO interrupts in unmask hook

The clearing was moved to the unmask hook because it is known to run
after the interrupt handler has actually run.  Before this patch, if
interrupts are threaded, the clearing/unmasking of level triggered
interrupts would be done before the threaded handler actually ran.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>