Convert to devm_gpio_request to save a few error handling code.

This patch properly handle the gpio_request failure with -EBUSY, we should
return error rather than ommit the gpio_request failure with -EBUSY.

I think one of the reason we got -EBUSY is because current code does not free
gpios in max8997_pmic_remove(). So it got -EBUSY when reload the module.

Yest another reason is in current code if gpio_request() returns -EBUSY,
the rest of the code still calls gpio_direction_output to config buck125_gpios
and set gpio value in max8997_set_gpio().  This looks wrong to me.
Signed-off-by: NAxel Lin <axel.lin@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>