brcmfmac: handle SDIO card removal

When removing the card the driver still tries to access registers
in the device. This patch adds another state for the bus that
indicates the device is no longer reachable. This avoids errors
accessing it while cleaning up the driver.

Reviewed-by: Franky Lin <frankyl@broadcom.com>
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>