snd_dummy: Bump primary channel count

Makes it easier to test scenarios involving more than 1 primary channels
per direction.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D59085

sys/dev/sound/dummy.c

@@ -45,8 +45,8 @@
 #include <mixer_if.h>
 #include <mpufoi_if.h>
 
-#define DUMMY_NPCHAN	1
-#define DUMMY_NRCHAN	1
+#define DUMMY_NPCHAN	2
+#define DUMMY_NRCHAN	2
 #define DUMMY_NCHAN	(DUMMY_NPCHAN + DUMMY_NRCHAN)
 
 struct dummy_chan {