Input: atmel_mxt_ts - fix double free of input device

[Nick Dyer: reworked to move free of input device into separate function
and only call in paths that require it.]
Signed-off-by: Nick Dyer <nick.dyer@itdev.co.uk>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>