brcmfmac: rename firmware_path to alternative_fw_path

In brcmfmac the module parameter "firmware_path" is used as an
alternative relative path under the search path used by firmware_class
or ueventhelper. Rename the parameter to alternative_fw_path to avoid
confusion.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>