brcmfmac: delete interface directly in code that sent fw request

brcmfmac: delete interface directly in code that sent fw request

So far when receiving event about in-firmware-interface removal our
event worker was notifying listener and afterwards it was removing Linux
interface.

First of all it was resulting in slightly unexpected order. The listener
(del_virtual_intf callback) was (usually) returning with success before
we even called unregister_netdev(ice).

Please note this couldn't be simply fixed by changing order of calls in
brcmf_fweh_handle_if_event as unregistering interface earlier could free
struct brcmf_if.

Another problem of current implementation are possible lockups. Focus on
the time slot between calling event handler and removing Linux
interface. During that time original caller may leave (unlocking rtnl
semaphore) *and* another call to the same code may be done (locking it
again). If that happens our event handler will stuck at removing Linux
interface, it won't handle another event and will block process holding
rtnl lock.

This can be simply solved by unregistering interface in a proper
callback, right after receiving confirmation event from firmware. This
only required modifying worker to don't unregister on its own if there
is someone waiting for the event.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>