gpio: omap: simplify read-modify-write

We already have a read-modify-write helper, but there's more that can
be done with a read-modify-write helper if it returned the new value.
Modify the existing helper to return the new value, and arrange for
it to take one less argument by having the caller compute the register
address.

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>