ath10k: clean up HTT tx buffer allocation and free

cleanup 'ath10k_htt_tx_alloc' by introducing the API's
'ath10k_htt_tx_alloc/free_{cont_txbuf, txdone_fifo} and
re-use them whereever needed

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>