Convert to devm_gpio_request to save a few error handling code.

This patch properly handle the gpio_request failure, we should return error
when gpio_request fails rather than just show warning.

I think one of the reason we got -EBUSY is because current code does not free
gpios in s5m8767_pmic_remove(). So it got -EBUSY when reload the module.

Yest another reason is in current code if gpio_request() returns error,
the rest of the code still calls gpio_direction_output to config buck_gpios
and buck_ds gpios. This looks wrong to me.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>