base-files: Do not break on non-eth ports

base-files: Do not break on non-eth ports

When using OpenWRT with DSA and 'lan' ports, we could get an empty
`next_eth`. This is of course not desirable, as this causes `sh: out of
range` errors when trying to determine which one would be greater.

It turns out, that we don't even need this check at all because, when
looking for all existin eth*s on a system, and take the highest index
and then iterate a set of devices and rename to eth${highest_index+n},
it is guaranteed that there will be no conflict.

Fixes: b688bf83f9d6 ("base-files: rename ethernet devs on known boards")
Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>