Input: exc3000 - factor out vendor data request

Factor out the vendor data i2c request handling to make it reusable
for other functions. Also don't cache the model and firmware version
string in the device private data as we never use the cached version,
but always read from the device.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20210125182527.1225245-3-l.stach@pengutronix.de


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>