Set speed for serial rigs only

[zcsahok]

Follow-up to #480: setting of serial speed for non-serial rigs fails due to internal checks in Hamlib function.

Rig model number is 2
Rig speed is 2400
Trying to start rig control
Speed not accepted!

Previously (when using low-level direct field setting) the value was simply unused and ignored. (2400 is the default speed in TLF)

The fix checks if we have a serial rig before setting the speed.
Inspired by Hamlib's src/conf.c:

...
    case TOK_SERIAL_SPEED:
        if (rp->type.rig != RIG_PORT_SERIAL)
        {
            return -RIG_EINVAL;
        }
...