jtag: parport: avoid freeing read-only memory section

jtag: parport: avoid freeing read-only memory section

If command parport_cable is not executed, parport_cable points to
const char array in read-only memory as default. On exit free()
will try to free this read-only memory. This patch uses strdup to
allocate memory when defining default setting.

Change-Id: I290e707ac6a37e9dc1b45c85ca51d8bd6aac6761
Signed-off-by: Stefan Mahr <stefan.mahr@sphairon.com>
Reviewed-on: http://openocd.zylin.com/1223
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>