jtag/drivers/ftdi: do not touch unavailable reset signals

jtag/drivers/ftdi: do not touch unavailable reset signals

If the current reset_config doesn't specify availability of nTRST or
nSRST, just leave them alone, do not try to deassert them ever
(asserting would be prevented by the upper layer).

Change-Id: I90123c666e05a1c26f1e164625e82d766a3e3744
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2186
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>