gpio: omap: simplify omap_set_gpio_irqenable()

omap_set_gpio_irqenable() calls two helpers that are almost the same
apart from whether they set or clear bits. We can consolidate these:

- in the set/clear bit register case, we can perform the operation on
  our saved context copy and write the appropriate set/clear register.
- otherwise, we can use our read-modify-write helper and invert enable
  if irqenable_inv is set.

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>