gpio: omap: irq_startup() must not return error codes

The irq_startup() method returns an unsigned int, but in __irq_startup()
it is assigned to an int.  However, nothing checks for errors, so any
error that is returned is ignored.

Remove the check for GPIO-input mode and the error return.

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>