usb: musb: debugfs: improve copy_from_user() argument

While the code is correct and functions well, it's still
a bit misleading to add the reference operator in from of
the buf argument.

This patch simply removes that operator in order to make
use of buf slightly better to the eyes.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>