gpio: em: Don't shadow error code of gpiochip_lock_as_irq()

gpiochip_lock_as_irq() may return a few error codes,
do not shadow them by -EINVAL and let caller to decide.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>