libertas: fix spelling mistake "Donwloading" -> "Downloading"

There is are two spelling mistakes in lbtf_deb_usb2 messages, fix these.

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>