regulator: gpio: properly check return value of of_get_named_gpio

The function of_get_named_gpio() could return -ENOENT, -EPROBE_DEFER
-EINVAL and so on. Currently, for the optional property "enable-gpio",
we only check -EPROBE_DEFER, this is not enough since there may be
misconfigured "enable-gpio" in the DTB, of_get_named_gpio() will return
-EINVAL in this case, we should return immediately here. And for the
optional property "gpios", we didn't check the return value, the driver
will continue to the point where gpio_request_array() is called, it
doesn't make sense to continue if we got -EPROBE_DEFER or -EINVAL here.

This patch tries to address these two issues by properly checking the
return value of of_get_named_gpio.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Mark Brown <broonie@kernel.org>