Input: atmel_mxt_ts - improve touchscreen size/orientation handling

Both T100 and T9 handle range and orientation in a similar fashion.
Reduce duplication between the two implementations.

Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>