brcmfmac: store usb fw images in local linked list.

For suspend/resume it is necessary to store firmware in memory.
In order to support multiple usb dongles at the same time a linked
list of firmwares was created.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>