cfg80211: merge documentations of field "dev"

The field "dev" was documented on two places. This patch merges the
comments.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200221115604.594035-8-Jerome.Pouiller@silabs.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>