usb: gadget: pxa27x: fix suspend callback

pxa27x disconnects pullups on suspend but doesn't
notify the gadget driver about it, so gadget driver
can't disable the endpoints it was using.

This causes problems on resume because gadget core
will think endpoints are still enabled and just
ignore the following usb_ep_enable().

Fix this problem by calling
gadget_driver->disconnect().

Cc: <stable@vger.kernel.org> # v3.10+
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>