usb: gadget: u_serial: use %*ph to print small buffer

Use %*ph format to print small buffer as hex string.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20201228200203.58525-1-andriy.shevchenko@linux.intel.com


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