Bamboos with Product ID's > 0xD4 return values unrelated to pressure
in touch 1 pressure field.  They also report 2nd touch X/Y values
shifted down 1 byte (where pressure was).  This results in jumpy
1 finger touch and totally invalid 2nd finger data.

For touch detection, switch to a Touch Present single bit that
all versions of Bamboo support.

For touch 2 offset, calculate offset based on a bit that is set
different between the two packet layouts.

Since touch pressure reports were removed from driver, there was
no need to be reading pressure any more.
Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Reviewed-by: Ping Cheng <pinglinux@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>