usb: composite: add bind_deactivated flag to usb_function

This patch introduces 'bind_deactivated' flag in struct usb_function.
Functions which don't want to be activated automatically after bind should
set this flag, and when they start to be ready to work they should call
usb_function_activate().

When USB function sets 'bind_deactivated' flag, initial deactivation
counter is incremented automatically, so there is no need to call
usb_function_deactivate() in function bind.

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