Count the MLO vdevs in a pdev. WLAN_VDEV_FEXT2_MLO feature
flag in vdev object is set for all MLO vdevs. Simple solution is
to increment/decrement the count on every set/clear of
WLAN_VDEV_FEXT2_MLO feature flag. Since this feature flag will be
set/clear at multiple places for a given vdev, this approach
will lead to wrong MLO vdev count. To fix this we need to
increment/decrement the count on first set/first clear of
WLAN_VDEV_FEXT2_MLO flag. Add a lock also to prevent the race
conditions.
CRs-Fixed: 3106235
Change-Id: Ice7edde04553088fbb7c9b769508d441ccd6e4bf
40dacfa471