mac80211: Fix Ptk0 rekey documentation

@IEEE80211_KEY_FLAG_GENERATE_IV setting is irrelevant for RX.
Move the requirement to the correct section in the PTK0 rekey
documentation.

Signed-off-by: Alexander Wetzel <alexander@wetzel-home.de>
Link: https://lore.kernel.org/r/20210924200514.7936-1-alexander@wetzel-home.de


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