fix(icom): verify the data flag after a mode set, and correct it with 0x26

The IC-7760 acknowledges the 1A 06 data-mode command and stays in USB-D
anyway, which left the operator unable to get back to plain USB from the
console at all. After every mode set the flag is read back; on a mismatch it
is said again with 0x26 — mode, data flag and filter in one frame, the form
the newer rigs actually honour. Only on a mismatch, so rigs that predate 0x26
never see the command.

internal/cat/civ/civ.go

@@ -36,6 +36,7 @@ const (
 	CmdVfoFreq        = 0x25 // read a specific VFO's freq (sub 0x00 selected, 0x01 unselected)
 	CmdPTT            = 0x1C // sub 0x00 = PTT
 	CmdExtra          = 0x1A // sub 0x06 = data mode on modern Icoms
+	CmdModeDataFil    = 0x26 // sub 0x00 = selected VFO: mode + data flag + filter in one frame
 	CmdReadID         = 0x19 // sub 0x00 = rig's own CI-V address (identifies model)
 	CmdPower          = 0x18 // power on/off (sub 0x01 = on, 0x00 = off; on needs an FE wake preamble)