extcon: Amend kernel documentation of struct extcon_dev

First of all, the @lock description is missing. Add it.
Second, correct the terminator value for the mutual exclusive
cabling.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>