meta-quanta: common: fix "ignored null byte in input" warning

get rid of "ignored null byte in input" warning for
the usb-network.sh

Signed-off-by: George Hung <george.hung@quantatw.com>
Change-Id: I2a18f568b362601fa1c41ec39a8c1cecf09f8862
diff --git a/meta-quanta/meta-common/recipes-quanta/network/usb-network/usb-network.sh b/meta-quanta/meta-common/recipes-quanta/network/usb-network/usb-network.sh
index 6840f9f..ebd6f02 100644
--- a/meta-quanta/meta-common/recipes-quanta/network/usb-network/usb-network.sh
+++ b/meta-quanta/meta-common/recipes-quanta/network/usb-network/usb-network.sh
@@ -12,7 +12,7 @@
 # Set the locally administered bit (the second least-significant
 # bit of the first octet) of the MAC address
 set_local_administered_bit() {
-    mac="$(cat $1)"
+    mac="$(tr -d '\0' < $1)"
     first_byte="${mac:0:2}"
     first_byte="$((0x$first_byte|2))"
     first_byte="$(printf "%02x\n" "$first_byte")"