usb: phy: qusb: Fix invalid gpio being treated as valid

A gpio of value 0 is valid and currently in the case
of invalid gpio, the notifier_gpio is updated to 0.
This causes invalid calls of get gpio value on gpio 0.

Fix this by not setting the gpio value to 0, if a valid
gpio could not be fetched.

Change-Id: Idbf3a36df6dfb4eaca1fe53d59faccc4b064d461
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>