ssb/gpio: Remove unnecessary WARN_ON from driver_gpio

The WARN_ON triggers on older BCM4401-B0 100Base-TX ethernet controllers.
The warning serves no purpose. So let's just remove it.

Reported-by: H Buus <ubuntu@hbuus.com>
Signed-off-by: Michael Büsch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>