brcmfmac: check bus state for status

Bus state should be the correct flag for bus status. Use it instead
of result from previous function call for backplane clock switch.

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