Input: wacom - include and use linux/hid.h

The current wacom code redefines constants that are already in linux/hid.h
This patch includes the official implementation and use it accross the code.

There is a conflict with HID_USAGE and others at the same level:
- in the wacom.ko implementation, those are the #define regarding the
  value of the field in the report descriptor
- in the hid.h, those are bitmask
So add HDESC_ in their current definition.

Also, the struct hid_descriptor slightly differs from the linux/hid.h
point of view, so mark it as custom for this driver.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>