extcon: Modify the device name as extcon[X] for sysfs

This patch modify the device name as extcon[X] for sysfs by using the 'extcon'
prefix word instead of separate device name. On user-space aspect, user would
find the some extcon drvier with extcon[X] pattern. So, this patch modify the
device name as following:
- /sys/class/extcon/[device name] -> /sys/class/extcon/extcon[X]

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>