Input: elantech - provide a sysfs knob for crc_enabled

The detection of crc_enabled is known to fail for Fujitsu H730. A DMI
blacklist is added for that, but it can be expected that other laptops will
pop up with this.

Here a sysfs knob is provided to alter the behaviour of crc_enabled.
Writing 0 or 1 to it sets the variable to 0 or 1. Reading it will show the
crc_enabled variable (0 or 1).

Reported-by: Stefan Valouch <stefan@valouch.com>
Signed-off-by: Ulrik De Bie <ulrik.debie-os@e2big.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>