usb: composite: fix usb_function_activate/deactivate functions

Using usb_gadget_disconnect to make gadget temporarily invisible to host
doesn't provide desired result, because gadget is connected immediately
after binding regardless to previous usb_gadget_disconnect() calls.

For this reason we use usb_gadget_deactivate() instead of
usb_gadget_disconnect() to make it working as expected.

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