gpio: omap: remove irq_ack method

The irq_ack method does not fit our hardware requirements. Edge
interrupts must be cleared before we handle them, and level interrupts
must be cleared after handling them.

We handle the interrupt clearance in our interrupt handler for edge IRQs
and in the unmask method for level IRQs.

Replace the irq_ack method with the no-op method from the dummy irq
chip.

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>