usb: gadget: composite: make sure to reactivate function on unbind

If a function sets bind_deactivated flag, upon removal we will be left
with an unbalanced deactivation. Let's make sure that we conditionally
call usb_function_activate() from usb_remove_function() and make sure
usb_remove_function() is called from remove_config().

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>