ath10k: initialize nbytes to 0

ath10k firmware checks nbytes == 0 as part of determining if DMA
has completed successfully.  To help make this work more often,
have the driver initialize nbytes to zero when freeing the descriptor
slot.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>