Input: atmel_mxt_ts - use T9 reportid range to init number of mt slots

Atmel mxt devices can report one finger for each T9 reportid.
Therefore, this range can be used to report the max number of MT-B slots
to userspace instead of assuming a fixed 10.

Note that mxt_initialized() must complete early, since the input_dev
properties now depend on values in the object table.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>