regulator: pfuze100: Delete reference to ena_gpio

We now pass a GPIO descriptor to the core instead of a global
GPIO number, if this descriptor is NULL the GPIO line is not
used. Just delete the assignment of an invalid GPIO line.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>