/etc/init.d/lldpd.sh: fix another bashism

This removes "printf -v <outputvar> <format> ..." which is only
available in bash.

These needless incompatible extensions to standard commands are a
source of endless headaches.

In this case, my previous "fix" to this file was incomplete, because
I didn't not the braindead "printf -v" command.

Also, this changes the shebang to call sh (which is already done by rcS,
only the interactive execution now uses sh instead of bash).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>