ath6kl: update Kconfig descriptions

The help text were either out of date, too small or didn't exist at all. Also
move cfg80211 dependency to ath6kl_core module as it has all the calls to
cfg80211.

Fixes checkpatch warning:

WARNING: please write a paragraph that describes the config symbol fully

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>