Input: wacom - prepare for syncing with input-mt changes

Henrik added new MT routines in release 3.7. This patch is to prepare
for the use of new MT routines.

In the newly added wacom_abs_set_axis() function, the first
if-statement assigns ABS_X/Y for number of contacts less or equal
to 2. So, it is single and 2 finger touch devices. Two finger touch
devices are processed here since they will not use the updated
input_mt_init_slots(), which does not offer benefit to those devices.

input_mt_init_slots() will take care of ABS_X/Y assignment when flags
are not zero. All touch devices with more than two contacts will use
input_mt_init_slots() with non-zero flags.

The second if-statement is for all MT devices, which include two
finger touch devices.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>