usb: mtu3: mtu3_debug: remove an unused struct member

The "nregs" member is not used.  The code uses the "regset.nregs"
struct member instead.

Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/X8ikv1QA3Do50D+R@mwanda


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