b43legacy: Fix spelling mistake "overflew" -> "overflowed"

There is a spelling mistake in a comment. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210601102855.8884-1-colin.king@canonical.com