setserial: bump to version 2.17-45.3

Bump to patchlevel 3 since 2 is gone upstream.
Also switch to use http://snapshot.debian.org/

 instead of BR2_DEBIAN_MIRROR to
avoid disappearing packages.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>