brcmfmac: make brcmf_fw_nvram_strip() static

brcmfmac: make brcmf_fw_nvram_strip() static

The function brcmf_fw_nvram_strip() is no longer called so
it does not need to be exposed.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@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>