sxmo_audio*: properly set vars with default values if they are unset

A mistake from implementing this resulted in the shell trying to expand
the 'word' part (e.g. "Line Out"), when the var was unset or null.
Obviously those are are strings and not expandable.

Signed-off-by: Anjandev Momi <anjan@momi.ca>

scripts/core/sxmo_audiocurrentdevice.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env sh
-${SPEAKER:-"Line Out"}
-${HEADPHONE:-"Headphone"}
-${EARPIECE:-"Earpiece"}
+SPEAKER=${SPEAKER:-"Line Out"}
+HEADPHONE=${HEADPHONE:-"Headphone"}
+EARPIECE=${EARPIECE:-"Earpiece"}
 
 audiodevice() {
 	amixer sget "$EARPIECE" | grep -qE '\[on\]' && echo "$EARPIECE" && return

scripts/core/sxmo_audioout.sh

@@ -5,9 +5,9 @@ ARG="$1"
 # shellcheck source=scripts/core/sxmo_common.sh
 . "$(dirname "$0")/sxmo_common.sh"
 
-${SPEAKER:-"Line Out"}
-${HEADPHONE:-"Headphone"}
-${EARPIECE:-"Earpiece"}
+SPEAKER="${SPEAKER:-"Line Out"}"
+HEADPHONE="${HEADPHONE:-"Headphone"}"
+EARPIECE="${EARPIECE:-"Earpiece"}"
 
 amixer set "$SPEAKER" mute
 amixer set "$HEADPHONE" mute