Hid: wacom: Fix kernel test robot warning

Kernel test robot throws the following warning -
>> drivers/hid/wacom_wac.c:2411:42: warning: format specifies type 'unsigned short' but the argument has type 'int' [-Wformat]
                           hid_warn(hdev, "Dropped %hu packets", value - wacom_wac->hid_data.sequence_number);
                                                   ~~~           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                   %d

Explicitly casting the argument to unsigned short to silence the warning and retain the intended behavior.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Joshua Dickens <joshua.dickens@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>