usb: dwc2: Add dwc2_gadget_dec_frame_num_by_one() function

Added dwc2_gadget_dec_frame_num_by_one() function in gadget.c.
This function will be used to calculate descriptor frame number field
value. For service interval mode frame number in descriptor should point
to last (u)frame in the interval.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>