ath6kl: Restrict memcpy to bounce buffer only for write request

No need to copy received local buffer content to bounce buffer
(DMA buffer) while performing sync READ operation from the chip.

It's applicable for only WRITE operation.

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>