HID: huion: Fix sparse warnings

Fix sparse warnings in hid-huion.c by using correct buffer type for
retrieved string descriptor.

The warnings in question were:

    drivers/hid/hid-huion.c:144:44: sparse: cast to restricted __le16
    drivers/hid/hid-huion.c:145:44: sparse: cast to restricted __le16
    drivers/hid/hid-huion.c:146:51: sparse: cast to restricted __le16
    drivers/hid/hid-huion.c:147:30: sparse: cast to restricted __le16

Reported-by:  <fengguang.wu@intel.com>
Signed-off-by: Nikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>