brcmfmac: Make firmware path a module parameter

This patch makes firmware path a module parameter so that firmware and
nvram files can be loaded from the specified path.

Signed-off-by: Daniel Kim <dekim@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>