brcmfmac: handle FWHALT mailbox indication

The firmware uses a mailbox to communicate to the host what is going
on. In the driver we validate the bit received. Various people seen
the following message:

 brcmfmac: brcmf_sdio_hostmail: Unknown mailbox data content: 0x40012

Bit 4 is cause of this message, but this actually indicates the firmware
has halted. Handle this bit by giving a more meaningful error message.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>