iio: accel: mma9551: Check gpiod_to_irq return value

The return value of gpiod_to_irq should be checked before giving
it to devm_request_threaded_irq in order to not pass an error
code in case it fails.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Reviewed-by: Vlad Dogaru <vlad.dogaru@intel.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>