brcmfmac: Protect against reprobing

It is important to reinitialise the firmware array pointers to protect
against the case that the brcmfmac driver is reprobed without first
being unloaded.

The potential hazard was noticed while investigating
https://github.com/raspberrypi/firmware/issues/1644

 .

Signed-off-by: Phil Elwell <phil@raspberrypi.com>