sandbox: eth-raw: Correct valid socket test in send/recv

In open, the socket is correctly checked to be -1 in the error case.
In send and recv, we checked for 0, but that is a valid socket number.

Correct this by checking for -1 as a bad socket everywhere.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>