usb: gadget: hid: Convert to use list_count()

The list API now provides the list_count() to help with counting
existing nodes in the list. Utilise it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20221123144901.40493-2-andriy.shevchenko@linux.intel.com


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