brcmfmac: Add support for first trying to get a board specific nvram file

The nvram files which some brcmfmac chips need are board-specific. To be
able to distribute these as part of linux-firmware, so that devices with
such a wifi chip will work OOTB, multiple (one per board) versions must
co-exist under /lib/firmware.

This commit adds support for callers of the brcmfmac/firmware.c code to
pass in a board_type parameter through the request structure.

If that parameter is set then the code will first try to load
chipmodel.board_type.txt before falling back to the old chipmodel.txt name.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>