ANDROID: GKI: fix up abi breakage from "cfg80211: fix management registrations locking"

Commit 4c22227e ("cfg80211: fix management registrations locking")
in 5.10.77, which is commit 09b1d5dc ("cfg80211: fix management
registrations locking") upstream, moved the mgmt_registrations_lock out
of struct wireless_dev.  This did not change the structure size, but
caused a CRC change in the structure pointer, which tripped the abi
checker.

So restore the variable, and comment that this is not used anymore.  No
driver should ever be touching this so it is safe.

Note, using a __GENKSYMS__ hack would work, as the overall structure
size remains the same when this variable is removed, BUT the warning
that the field is gone is reported by libabigail correctly, which would
require a .xml update, so it's not worth it.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ibcfe743833860e6f75611d7ebed1036a77be0a69