gpio: omap: simplify bank->level_mask

bank->level_mask is merely the bitwise or of the level detection
context which we have already read in this function.  Rather than
repeating additional reads, compute it from the values already
read.

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>