brcmfmac: restructure handling of IF event

The IF event need special care. It can be either an ADD, DEL, or
CHANGE. For an ADD we need to call brcmf_add_if() before the
event handler call. Upon a DEL we need to call brcmf_del_if()
after the event handler call. CHANGE does not require special
attention.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>