gpiolib: cdev: Fix kernel doc for struct line

Kernel doc validator is not happy:
  gpiolib-cdev.c:487: warning: Function parameter or member 'hdesc' not described in 'line'
  gpiolib-cdev.c:487: warning: Function parameter or member 'raw_level' not described in 'line'
  gpiolib-cdev.c:487: warning: Function parameter or member 'total_discard_seq' not described in 'line'
  gpiolib-cdev.c:487: warning: Function parameter or member 'last_seqno' not described in 'line'

Describe above mentioned parameters.

Fixes: 2068339a

 ("gpiolib: cdev: Add hardware timestamp clock type")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Dipen Patel <dipenp@nvidia.com>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thierry Reding <treding@nvidia.com>