HID: wacom: Limit touchstrip data to 13 bits

Commit c7f0522a uses sixteen bits of data in the construction of 'strip1'
and 'strip2'. This can cause problems in some cases, however, since some
tablets store flags in the MSB of data[2] and data[4] that should not be
included in these values. This restores the 0x1f mask that used prior
to c7f0522a.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>