ath10k: htt: don't use txdone_fifo with SDIO

HTT High Latency (ATH10K_DEV_TYPE_HL) does not use txdone_fifo at all, we don't
even initialise it by skipping ath10k_htt_tx_alloc_buf() in
ath10k_htt_tx_start(). Because of this using QCA6174 SDIO
ath10k_htt_rx_tx_compl_ind() will crash when it accesses unitialised
txdone_fifo. So skip txdone_fifo when using High Latency mode.

Tested with QCA6174 SDIO with firmware WLAN.RMH.4.4.1-00007-QCARMSWP-1.

Co-developed-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Alagu Sankar <alagusankar@silex-india.com>
Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>