gpio: omap: check gpiochip_add() return value

The gpiochip_add() function can fail if the chip cannot
be registered so the return value has to be checked and
the error propagated in case it happens.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>