Only call cmsis_dap_cmd_DAP_SWD_Configure when swd_mode is enabled

Only call cmsis_dap_cmd_DAP_SWD_Configure when swd_mode is enabled

The CMSIS-DAP used by NXP's LS1012ARDB board only supports JTAG,
and not SWD. Calling cmsis_dap_cmd_DAP_SWD_Configure returns with an
error (and doesn't actually do anything in the debugger).

Wrap the call to cmsis_dap_cmd_DAP_SWD_Configure in a check for
swd_mode, to make sure initialisation doesn't fail needlessly.

Change-Id: Id7e568cb6e36886bd7c5b3699d198a77a51c28c9
Signed-off-by: Bas Vermeulen <bas@daedalean.ai>
Reviewed-on: http://openocd.zylin.com/4294
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>