pwm: samsung: Document new member .channel in struct samsung_pwm_chip

pwm: samsung: Document new member .channel in struct samsung_pwm_chip

My earlier commit reworking how driver data is tracked added a new
member to struct samsung_pwm_chip but failed to add matching
documentation. Make up leeway.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310130404.uQ33q5Dk-lkp@intel.com/
Fixes: e3fe982b2e4e ("pwm: samsung: Put per-channel data into driver data")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>