Input: atmel_mxt_ts - don't report zero pressure from T9

If T9.CTRL DISAMP is set, then pressure is reported as zero. This means
some app layers (eg tslib) will ignore the contact.

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