brcmfmac: rename nvram.[ch] for upcoming firmware handling functions

The firmware processing will be modified to use asynchronous request
firmware api. In preparation this patch is simple rename of source
and header file to which the functionality will be added.

Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>