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.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>