dmaengine: usb-dmac: Avoid format-overflow warning

dmaengine: usb-dmac: Avoid format-overflow warning

gcc points out that the fix-byte buffer might be too small:
drivers/dma/sh/usb-dmac.c: In function 'usb_dmac_probe':
drivers/dma/sh/usb-dmac.c:720:34: warning: '%u' directive writing between 1 and 10 bytes into a region of size 3 [-Wformat-overflow=]
  720 |         sprintf(pdev_irqname, "ch%u", index);
      |                                  ^~
In function 'usb_dmac_chan_probe',
    inlined from 'usb_dmac_probe' at drivers/dma/sh/usb-dmac.c:814:9:
drivers/dma/sh/usb-dmac.c:720:31: note: directive argument in the range [0, 4294967294]
  720 |         sprintf(pdev_irqname, "ch%u", index);
      |                               ^~~~~~
drivers/dma/sh/usb-dmac.c:720:9: note: 'sprintf' output between 4 and 13 bytes into a destination of size 5
  720 |         sprintf(pdev_irqname, "ch%u", index);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Maximum number of channels for USB-DMAC as per the driver is 1-99 so use
u8 instead of unsigned int/int for DMAC channel indexing and make the
pdev_irqname string long enough to avoid the warning.

While at it use scnprintf() instead of sprintf() to make the code more
robust.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20240110222210.193479-1-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>