mac80211: refactor SKB queue processing a bit

This is a very long loop body, move it into its own function
instead, keeping only the kcov and free outside in the loop
body.

Link: https://lore.kernel.org/r/20210517230754.6bc6cdd68570.I28a86ebdb19601ca1965c4dc654cc49fc1064efa@changeid


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