mac80211: Add stations iterator where the iterator function may sleep

ieee80211_iterate_active_interfaces() and
ieee80211_iterate_active_interfaces_atomic() already exist, where the
former allows the iterator function to sleep. Add
ieee80211_iterate_stations() which is similar to
ieee80211_iterate_stations_atomic() but allows the iterator to sleep.
This is needed for adding SDIO support to the rtw88 driver. Some
interators there are reading or writing registers. With the SDIO ops
(sdio_readb, sdio_writeb and friends) this means that the iterator
function may sleep.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/20211228211501.468981-2-martin.blumenstingl@googlemail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>