In case of requests queue is empty reset EP target_frame to
initial value.

This allow restarting ISOC traffic in case when function
driver queued requests with interruptions.
Tested-by: NZeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: NMinas Harutyunyan <hminas@synopsys.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>