usb: gadget: pxa25x: fix disconnect reporting

when commit 6166c24 (usb: gadget: pxa25x_udc:
convert to udc_start/udc_stop) converted
this driver to udc_start/udc_stop, it failed
to consider the fact that stop_activity()
is called from disconnect interrupt.

Fix the problem so that gadget drivers know
about proper disconnect sequences.

Signed-off-by: Felipe Balbi <balbi@ti.com>