jtag: cmsis-dap: Issue disconnect before reconnecting

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>