HID: lenovo: Add middleclick_workaround sysfs knob for cptkbd

Previous attempt to autodetect well-behaving patched firmware
introduced in commit 46a0a2c9 ("HID: lenovo: Detect quirk-free fw
on cptkbd and stop applying workaround") has shown that there are
false-positives on original firmware (on both 1st gen and 2nd gen
keyboards) which causes the middle button click workaround to be
mistakenly disabled.

This commit adds explicit parameter to sysfs to control this
workaround.

Fixes: 46a0a2c9 ("HID: lenovo: Detect quirk-free fw on cptkbd and stop applying workaround")
Fixes: 43527a00 ("HID: lenovo: Restrict detection of patched firmware only to USB cptkbd")
Signed-off-by: Mikhail Khvainitski <me@khvoinitsky.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>