libertas: Fix misnaming for function param 'device'

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

 drivers/net/wireless/marvell/libertas/firmware.c:134: warning: Function parameter or member 'device' not described in 'lbs_get_firmware_async'
 drivers/net/wireless/marvell/libertas/firmware.c:134: warning: Excess function parameter 'dev' description in 'lbs_get_firmware_async'

Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: libertas-dev@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
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/20200819072402.3085022-27-lee.jones@linaro.org