test/dm: Fix string handling issues in the eth test

Coverity scan has identified potential buffer overruns in these tests.
Correct this by zeroing our buffer and using strncpy not strcpy.

Reported-by: Coverity (CID: 155462, 155463)
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>