ftdi: demote unhelpful debug messages

ftdi: demote unhelpful debug messages

Some protocols make use of empty scan fields for optional padding, which
causes the log to fill with unhelpful messages that a field is empty.
The remaining LOG_DEBUG messages in ftdi_execute_scan have been demoted
to DEBUG_JTAG_IO such that these messages are only seen when debugging
JTAG.

Change-Id: I61fd4551411ce851da34e67d003bca5d7a71cd92
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4112
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>