USB: gadget: net2280: remove leftover driver->unbind call in error pathway

This patch (as1667) removes an incorrect driver->unbind() call from
the net2280 driver.  If startup fails, the UDC core takes care of
unbinding the gadget driver automatically; the controller driver
shouldn't do it too.

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