gpio/omap: free irq domain in probe() failure paths

Currently the IRQ domain is not freed once allocated, in the case where
omap_gpio_probe() fails. Therefore, ensure we free the domain if the
probe does fail. Furthermore, the local variable "ret" is not needed
and so remove this.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>