package/netplug: fix quoting of ${NETWORKING} in init script

Since 4adaa581, S29netplug looks for
/etc/default/network instead of /etc/sysconfig/network.  When this
file exists but does not define $NETWORKING, the script fails on line
29 with something like:

/etc/init.d/S29netplug: 29: [: =: unexpected operator

Fix quoting so this error no longer happens.

Signed-off-by: Thomas Claveirole <thomas.claveirole@green-communications.fr>
[Thomas: keep double quotes around "no", keep curly braces when
referencing the variable.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>