regulator: gpio-regulator: gpio_set_value should use cansleep

If it's possible for gpio_set_value to sleep, we should be using
the *_cansleep call instead. This patch fixes multiple warnings
from gpiolib.

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>