regulator: pfuze100: Don't allocate an invalid gpio

Previously, the PFUZE100 would try to allocate gpio0 io0 because
config.ena_gpio defaults to 0, which can be a valid GPIO.  To prevent this
from happening, set this parameter to -EINVAL.

Signed-off-by: Sean Cross <xobs@kosagi.com>
Signed-off-by: Mark Brown <broonie@linaro.org>