extcon: Fix kernel doc of property fields to avoid warnings

Kernel documentation has to be synchronized with a code, otherwise
the validator is not happy:

     Function parameter or member 'usb_propval' not described in 'extcon_cable'
     Function parameter or member 'chg_propval' not described in 'extcon_cable'
     Function parameter or member 'jack_propval' not described in 'extcon_cable'
     Function parameter or member 'disp_propval' not described in 'extcon_cable'

Describe the fields added in the past.

Fixes: 067c1652 ("extcon: Add the support for extcon property according to extcon type")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>