usb: dwc2: add printf attribute to cat_printf()

As cat_printf() uses printf format strings in its parameters, adding
__printf attribute allows the compiler to detect at compile-time some
errors related to format strings (with -Wformat warning flag).

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>