brcmfmac: firmware: Demote seemingly unintentional kernel-doc header

brcmfmac: firmware: Demote seemingly unintentional kernel-doc header

The function parameter is not documented and either are any of the
other functions in this source file.

Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/broadcom/brcm80211/brcmfmac/firmware.c:69: warning: Function parameter or member 'c' not described in 'is_nvram_char'

Cc: Arend van Spriel <arend.vanspriel@broadcom.com>
Cc: Franky Lin <franky.lin@broadcom.com>
Cc: Hante Meuleman <hante.meuleman@broadcom.com>
Cc: Chi-Hsien Lin <chi-hsien.lin@cypress.com>
Cc: Wright Feng <wright.feng@cypress.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: linux-wireless@vger.kernel.org
Cc: brcm80211-dev-list.pdl@broadcom.com
Cc: brcm80211-dev-list@cypress.com
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200821071644.109970-13-lee.jones@linaro.org