brcmfmac: rework usb callback operations

The resume callbacks do partly the same a the probe callback
so put common code in separate function for use in the callbacks.
This also fixes suspend/resume regression introduced by

    brcmfmac: remove .init() callback for internal bus interface

    The .init() callback was the first function called by the common
    bus function brcmf_bus_start(). Given that it is not really
    necessary and the bus layer can call it before calling the
    brcmf_bus_start() function.

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: John W. Linville <linville@tuxdriver.com>