HID: wacom: Fix pad button range for CINTIQ_COMPANION_2

Commit c7f0522a incorrectly constructs the 'buttons' variable for the
CINTIQ_COMPANION_2 case. The high nybble of data[2] is shifted four
bits too far, leaving the bits associated with BTN_7 through BTN_A
unset.

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