usb: dwc3: gadget: fix a kernel-doc warning

The multiplier parameter of dwc3_gadget_calc_tx_fifo_size() was
not documented:

	drivers/usb/dwc3/gadget.c:675: warning: Function parameter or member 'mult' not described in 'dwc3_gadget_calc_tx_fifo_size'
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/bd599a18cea45c57d91c69d3e30d8b1e8ea69dd1.1656409369.git.mchehab@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/gadget.c

@@ -657,6 +657,7 @@ static int dwc3_gadget_set_ep_config(struct dwc3_ep *dep, unsigned int action)
 /**
  * dwc3_gadget_calc_tx_fifo_size - calculates the txfifo size value
  * @dwc: pointer to the DWC3 context
+ * @mult: multiplier to be used when calculating the fifo_size
  *
  * Calculates the size value based on the equation below:
  *