ath10k: replace send_head() with tx_sg()

PCI is capable of handling scatter-gather lists.
This can be used to avoid copying memory.

Change the name of the callback while at to
reflect its purpose.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>