ath6kl: Avoid rolling back of entire scatter setup in case of failure

Current tx scatter gather implementation rolls back the entire
scatter setup in case of a failure in setting up just one packet
into the bundle. Instead of dopping the whole scatter setup,
send the packets available just before the failure one using
scatter gather I/O.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>