wl1251: cmd: Rename 'len' to 'buf_len' in the documentation

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

 drivers/net/wireless/ti/wl1251/cmd.c:70: warning: Function parameter or member 'buf_len' not described in 'wl1251_cmd_test'
 drivers/net/wireless/ti/wl1251/cmd.c:70: warning: Excess function parameter 'len' description in 'wl1251_cmd_test'

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: 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/20201102112410.1049272-10-lee.jones@linaro.org