ath11k: set WMI pipe credit to 1 for QCA6390

For QCA6390, set wmi credit to 1 to avoid back-to-back write to
shadow register when shadow register is enabled.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1
Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1601544890-13450-4-git-send-email-kvalo@codeaurora.org

drivers/net/wireless/ath/ath11k/htc.c

@@ -515,6 +515,12 @@ int ath11k_htc_wait_target(struct ath11k_htc *htc)
 		return -ECOMM;
 	}
 
+	/* For QCA6390, wmi endpoint uses 1 credit to avoid
+	 * back-to-back write.
+	 */
+	if (ab->hw_params.supports_shadow_regs)
+		htc->total_transmit_credits = 1;
+
 	ath11k_htc_setup_target_buffer_assignments(htc);
 
 	return 0;