extcon: Change permission 'state' sysfs entry (rw -> r)

This patch change permission from read/write to only read.
The specific process in the user-space couldn't change the state
of cable when cable is attached or detached.
- /sys/class/extcon/[devine name]/state

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>