Add spilt mode get and set for Elecraft K3.
Fix data sub-modes on Elecraft K3.

The K3 supports AFSK & FSK sub-modes and for the D versions it also
has an internal RTTY and PSK31 decoder. The decoder sub-modes are
reported as FSK (RTTY) and the AFSK sub-modes are reported as PKT(USB
& LSB). LSB modes are assumed to be RTTY and USB modes are assumed to
be PKT(PSK, WS modes etc.).

For mode set the data sub-modes are set as follows:

RTTY -> FSK D normal (LSB) VFO shows MARK QRG
RTTYR -> FSK D reversed (USB) VFO shows MARK QRG
PKTUSB -> DATA A normal (USB) VFO shows suppressed carrier QRG
PKTLSB -> AFSK A normal (LSB) optimised for RTTY VFO shows MARK QRG

Not all data sub-mode combinations are possible but the above mapping
seems most likely to cover the user requirements.
Fix Elecraft Data Mode Translation.

Ensure a valid mode is set for all possible mode set values.
Fix Elecraft K3 Data Sub-Mode Setting.

The data sub-mode has to be set after setting the bandwidth on the
"back" VFO since the BW$ command seems to reset the data sub-mode.