HID: length resolution should be reported units/mm

Input ABI requires reporting resolution on main axes in units per
millimeter, not units per inch, so we need to convert accordingly.

Tested-by: Nikolai Kondrashov <spbnick@gmail.com>
Acked-by: Nikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>