wifi: brcmfmac: Fix a typo "unknow"

It should be "unknown".

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20221101170252.1032085-1-j.neuschaefer@gmx.net