Expose model and fw_version via sysfs. Also query the model
in probe to make sure, that the I2C communication with the
device works before successfully probing the driver.

This is a bit complicated, since EETI devices do not have
a sync interface. Sending the commands and directly reading
does not work. Sending the command and waiting for some time
is also not an option, since there might be touch events in
the mean time.

Last but not least we do not cache the results, since this
interface can be used to check the I2C communication is still
working as expected.
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20200805160520.456570-5-sebastian.reichel@collabora.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>