Per gpio_chip interface, error shall be proparated to the caller.

Attempt to silent diagnostics by returning zero (as written in the
comment) is plain wrong, because the zero return can be interpreted by
the caller as the gpio value.

Cc: stable@vger.kernel.org
Signed-off-by: Andrey Gusakov <andrey.gusakov@cogentembedded.com>
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>