[brcmfmac] Stablize iface recovery

A race condition between wlanphy and wlanif's interface shutdown paths
resulted in an unstable iface recovery process. This change delays the
lock release until brcmfmac-wlan-fullmac-client device is fully
removed. This is a workaround, the most elegant solution should be
modifying the iface recovery trigger point, see detail in the bug
comment.

Bug: 124933
Test: In core build, delete the client iface for 100 times and able proceed scan with
the iface number 100.

Change-Id: Idc50be4792a6121354f4109e97a9fc3b4fd492b8
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/834280
Commit-Queue: Zhiyi Chen <zhiyichen@google.com>
Reviewed-by: John Tan <johntan@google.com>