input: atmel_mxt_ts: Zero out config version after firmware upgrade

When the touch controller firmware is upgraded, the config data on
the controller might require an update too to match the firmware.
Since the driver performs config version checking before updating
config data and only updates to a newer version, we need to zero
out the config version after a firmware upgrade to make sure proper
config data will be programmed to the touch controller.

Change-Id: If3e22948b1f8eea3ccbfcaa979ec6b42f1ac078a
Signed-off-by: Jing Lin <jinglin@codeaurora.org>