extcon: make extcon_info static const, fixes warning

The array extcon_info is read only, local to the source and does not
need to be in global scope, so make it static const.

Cleans up sparse warning:
symbol 'extcon_info' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>