Fix checkstyle invocation for l2switch builds

I pushed a change to l2switch project recently (https://git.opendaylight.org/gerrit/#/c/35300/) which accidentally had tabs in it. The build went through fine without checkstyle complaining about it (on other projects e.g. bgpcep, build would fail)

Looks like l2switch is setup to use checkstyle during build, but it has got broken. Opening this bug to get it fixed