jtag: cmsis-dap: Issue disconnect before reconnecting

cmsis-dap protocol has both DAP_Connect and DAP_Disconnect commands.
Logically if cmsis_dap_swd_switch_seq() calls DAP_Connect in connected
state it should call DAP_Disconnect first.

Doing so surprisingly solves problems on Atmel EDBG with target SAMD/R/L/C.
Without this change SWD communication breaks after reset run/halt
in config "reset_config srst_only" and reconnect trials repeatedly
fail with "SWD ack not OK: 0 JUNK"

Change-Id: Ie951098d5e0c83f388e2df414608aaabec2135c9
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3098
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/cmsis_dap_usb.c

@@ -714,6 +714,9 @@ static int cmsis_dap_swd_switch_seq(struct adiv5_dap *dap, enum swd_special_seq
 	unsigned int s_len;
 	int retval;
 
+	/* First disconnect before connecting, Atmel EDBG needs it for SAMD/R/L/C */
+	cmsis_dap_cmd_DAP_Disconnect();
+
 	/* When we are reconnecting, DAP_Connect needs to be rerun, at
 	 * least on Keil ULINK-ME */
 	retval = cmsis_dap_cmd_DAP_Connect(seq == LINE_RESET || seq == JTAG_TO_SWD ?