USB: net2280: Remove ->disconnect() callback from net2280_pullup()

The net2280 UDC driver invokes the gadget driver's ->disconnect()
callback routine when the net2280_pullup() routine turns off the D+
pullup.  This is now unnecessary, because the gadget core performs the
callback on our behalf.  This patch removes the unneeded callback.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>