msi-laptop: Add threeg sysfs file for support query 3G state by standard 66/62 ec command

Add threeg sysfs file for support query 3G state by standard 66/62 ec
command, the MSI standard ec interface supported this feature.

Signed-off-by: Lee, Chun-Yi <jlee@novell.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>