adi_v5_swd: Read RDBUFF once after a sequence of AP reads

Increases performance by a factor of two for long reads.

Change-Id: I81a7a83835058560c6a53a43c3cc991100f01766
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1954
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

src/target/arm_adi_v5.c

@@ -661,6 +661,7 @@ int ahbap_debugport_init(struct adiv5_dap *dap)
 	 */
 	dap->ap_current = !0;
 	dap_ap_select(dap, 0);
+	dap->last_read = NULL;
 
 	/* DP initialization */