usb: gadget: add usb_gadget_activate/deactivate functions

These functions allows to deactivate gadget to make it not visible to
host and make it active again when gadget driver is finally ready.

They are needed to fix usb_function_activate() and usb_function_deactivate()
functions which currently are not working as usb_gadget_connect() is
called immediately after function bind regardless to previous calls of
usb_gadget_disconnect() function.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>