USB-Audio: Scarlett 18i20 gen4 - improve channel detection

Use bcdDevice number (using sysfs) to detect the right
channel count. Information is taken from the pull request
comments bellow.

Link: alsa-project#660
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

Author: perexg

ucm2/USB-Audio/Focusrite/Scarlett-18i20.conf

@@ -1,4 +1,4 @@
-# The 18i20 provides 20 playback and 18 or 20 capture channels each:
+# Channel mapping
 #
 # +----+---------------+-----------------+
 # | Ch |     Inputs    |      Outputs    |
@@ -100,9 +100,31 @@ If.gen4 {
 		String "${CardComponents}"
 		Regex "USB1235:821d"
 	}
-	True.Define {
-		Generation "4th"
-		DirectCaptureChannels 20
+	True {
+		Define {
+			Generation "4th"
+			DirectPlaybackChannels 26
+			DirectCaptureChannels 26
+			bcdDevice "${sys-card:device/../bcdDevice}"
+		}
+		If.fwp {
+			# bcdNumbers 0900-097a
+			Condition {
+				Type RegexMatch
+				String "${var:bcdDevice}"
+				Match "(09[0-6].|097[0-9a])"
+			}
+			True.Define.DirectPlaybackChannels 24
+		}
+		If.fwc {
+			# bcdNumbers 0000-08ff, 0900-097a
+			Condition {
+				Type RegexMatch
+				String "${var:bcdDevice}"
+				Match "(0[0-8]..|09[0-6].|097[0-9a])"
+			}
+			True.Define.DirectCaptureChannels 20
+		}
 	}
 }