dahdi_span_types: hush warning of missing attribute

Don't warn when a sysfs attribute does not exist.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
Acked-By: Russ Meyerriecks <rmeyerriecks@digium.com>

dahdi_span_types

@@ -166,7 +166,7 @@ fi
 
 # Beware of special characters in attributes
 attr_clean() {
-	cat "$1" | tr -d '\n' | tr '!' '/' | tr -c 'a-zA-Z0-9/:.-' '_'
+	cat "$1" 2>/dev/null | tr -d '\n' | tr '!' '/' | tr -c 'a-zA-Z0-9/:.-' '_'
 }
 
 show_spantypes() {