update-rc.d: Fix enable/disable command

Now works for any number 01-99 in both bash and busybox sh.

$[] is a bashism and does not work on POSIX shells.
Links numbered 91-99 created a single digit link.
Links numbered 08 and 09 gave the error:
'bash: 100-08: value too great for base (error token is "08")'

Signed-off-by: Einar Gunnarsson <tolvupostur@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>