systemd-conf-canbus: Add link configuration

Add a .link file to set the transmit queue length to 1000 for physical
CAN devices.  This matches the default used for vcan devices, and in
testing fixes the transmit buffer overflows when running the simulator
script against an interface.

Bug-AGL: SPEC-4652

Change-Id: Icb190cdcbda9b09e337789d3a88e39f7288f34eb
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28291
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account