udev: network-hotplug-bridges: Silence a warning when interfaces have gone away

udev: network-hotplug-bridges: Silence a warning when interfaces have gone away

It can happen that udev has an event for an interface in the queue that
has already gone away - or even just being renamed.

Then reading the MAC address fails. Because the shell expands the
"$(<...)" statement before running the whole line, the read check is
useless.

Because the code would get too complicated otherwise, I decided to use
cat. Not cool, but this does the job.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>