iwlegacy: fix spelling mistake "acumulative" -> "accumulative"

Fix spelling mistakes in rx stats text. I missed these from an earlier
round of fixing the same spelling mistake.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>