usb: gadget: pch_udc: Fix disconnect issue

ISSUE:
When the driver notifies a gadget of a disconnect event, a system
rarely freezes.

CAUSE:
When the driver calls dev->driver->disconnect(), it is not calling
spin_unlock().

Signed-off-by: Tomoya MORINAGA <tomoya.rohm@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>