usb: gadget: f_obex: use bind_deactivated flag

Use bind_deactivated flag instead of calling usb_function_deactivate()
in function bind(). Field 'can_activate' in struct f_obex is no longer
needed as setting 'bind_deactivated' flag makes us sure, that the function
will be binded only if deactivation can be performed successfully.

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