drivers/ftdi: warn the user if he tries to use undefined reset signals

drivers/ftdi: warn the user if he tries to use undefined reset signals

It is really an error to try to use a reset_config that doesn't match
adapter's capabilities, however OpenOCD has no way to specify them.

Using wrong reset_config might lead to very confusing behaviour, so I
think LOG_ERROR is justified here.

Change-Id: I1c6dcfa7c0d78829229a850189cad646b565dd66
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1948
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>