jtag/drivers/cmsis-dap: fix connect under reset

jtag/drivers/cmsis-dap: fix connect under reset

Commit ef02b69b14d133b061217a91add5a028a77e86bc included
a call to cmsis_dap_cmd_DAP_Connect() before calling
cmsis_dap_cmd_DAP_SWJ_Sequence(). According to comment
it is necessary for at least Keil ULINK-ME.

Commit 72c3464be42088dc75245cf2fcc8f5c6e6959b4b added
a cmsis_dap_cmd_DAP_Disconnect() before connect call to pair
connection/disconnection. It solves some problems on Atmel EDBG.

Unfortunately calling either of cmsis_dap_cmd_DAP_Connect()
or cmsis_dap_cmd_DAP_Disconnect() deasserts reset signal.
So these workarounds break ability to connect under reset.

Use cmsis_dap_cmd_DAP_Disconnect() and cmsis_dap_cmd_DAP_Connect()
pair only if both SRST and TRST are deasserted.

Change-Id: I0914dae0a1360b8c7fe48231ff3867caedfb2dbe
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reported-by: Leonardo Sabino dos Santos <leonardo.sabino@gmail.com>
Reviewed-on: http://openocd.zylin.com/4100
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>