staging: most: hdm-usb: introduce synchronization function

This patch introduces the function start_sync_ep() and relocates the
triggers for synchronization to this function.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>