brcm80211: fmac: change firmware/nvram name to be more generic

The nvram file contains info for firmware which varies with different
hardware designs. Use more common firmware/nvram file names instead
of those in Linux firmware repository to avoid misunderstanding.

Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>