manager: Add brcmfmac to the blacklist

brcmfmac does not allow the removal of the default / primary interface.
So there isn't much point in having iwd attempt this.

Another issue is that brcmfmac _does_ allow the deletion of non-default
interfaces.  So starting iwd on a system with a station & ap interface
active can result in iwd attempting to delete all the interfaces.  Given
the above, it succeeds in deleting the ap interface but not the station
one.  In strange circumstances it might end up thinking that the ap
interface is the 'default' and trying to use it, whereas it was just
successfully removed.

src/manager.c

@@ -80,6 +80,7 @@ static const char *default_if_driver_list[] = {
 	"rtl87*",
 	"rtl88*",
 	"rtw_*",
+	"brcmfmac",
 
 	NULL,
 };