usb: phy: fix error handling in usb_get_phy

spin_unlock_irqrestore() was not being called in the error path of
usb_get_phy. It's fixed here.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>