In case of a missing (optional) gpio don't fall through up to
"ti,active-discharge-time-us" due to
devm_fwnode_get_index_gpiod_from_child() returning NULL (since
gpiod_get_from_of_node() returned NULL) but rather indicate success as
intended.

This makes the driver probe correctly when e.g. only the enable gpio is
given.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Link: https://lore.kernel.org/r/363bd50cc7c60daa57d614a341d1fd649f05194c.1567625660.git.agx@sigxcpu.orgSigned-off-by: Mark Brown <broonie@kernel.org>