Input: usbtouchscreen - add sysfs attribute for 3M MTouch firmware rev

Allow querying of the firmware revision of the device

example:

4.10

Tested on ZII RDU2 platform and on Intel x86_64 PC.

Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>