hameg-hmo: support triggering on either falling or rising edge

At least the HMO 1002 allows triggering on rising or falling edge, the
corresponding SCPI value is "EITH"

See also bug #740

src/hardware/hameg-hmo/protocol.c

@@ -78,6 +78,7 @@ static const char *hmo_coupling_options[] = {
 static const char *scope_trigger_slopes[] = {
 	"POS",
 	"NEG",
+	"EITH",
 	NULL,
 };