Add support for Data Modes on Icom "Pro" models.

The Icom IC-7x6Pro models support AFSK data modes with Tx audio
delivered via the back ACC socket. This mode is enabled and queried
via a separate "0x1A06" command and applies to USB, LSB, FM and, AM as
USB-D, LSB-D, FM-D and, AM-D respectively on the radio. I have added
all but AM because hamlib has no mode enumeration for AM Data mode.

Most of the IC-7xxx rigs have an extended version of this special
command which also involves width. This needs further work to
implement.

Signed-off-by: Nate Bargmann <n0nb@n0nb.us>