Input: atmel_mxt_ts - zero terminate config firmware file

We use sscanf to parse the configuration file, so it's necessary to zero
terminate the configuration otherwise a truncated file can cause the
parser to run off into uninitialised memory.

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