gpio/gpio-omap: Use existing pointer to struct device

A pointer to "pdev->dev" is already stored in "dev", so use it in
devm_kzalloc.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>