usb: dwc2: gadget: use existing helper

All other checks of params.g_dma_desc in this file use the
using_desc_dma() helper function.  Use that helper in two more places
for consistency throughout the file.

Acked-by: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Signed-off-by: John Keeping <john@metanate.com>
Link: https://lore.kernel.org/r/20211122123107.3147353-1-john@metanate.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>