gpio: omap: clean up edge interrupt handling

The edge interrupt handling was effectively:

	isr = ISR_reg & enabled;
	if (bank->level_mask)
		level_mask = bank->level_mask & enabled;
	else
		level_mask = 0;

	edge = isr & ~level_mask;

When bank->level_mask is zero, level_mask will be computed as zero
anyway, so the if() statement is redundant.  We are then left with:

	isr = ISR_reg & enabled;
	level_mask = bank->level_mask & enabled;
	edge = isr & ~level_mask;

This can be simplified further to:

	isr = ISR_reg & enabled;
	edge = isr & ~bank->level_mask;

since the second mask with 'enabled' is redundant.

Improve the associated comment as well.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>