Input: wacom - handle 1024 pressure levels in wacom_tpc_pen

Some tablet PC sensors (e.g. the 0xEC found in the Thinkpad Yoga) report
more than 256 pressure levels and will experience wraparound unless the
full range is read.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Aaron Skomra <Aaron.Skomra@wacom.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>