wifi: brcmfmac: clarify unmodifiable headroom log message

Replace misleading log "insufficient headroom (0)" with more clear
"unmodifiable headroom".

Signed-off-by: Alex Shumsky <alexthreed@gmail.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241213081402.625003-1-alexthreed@gmail.com