sandbox: eth-raw-os.c: Ensure that our interface name is not too long

sandbox: eth-raw-os.c: Ensure that our interface name is not too long

Coverity notes that we do not ensure when we copy ifname we still have
space left to ensure NULL termination.  As cannot control the size of
ifr_name we must make sure that our argument will not overflow the
buffer.

Reported-by: Coverity (CID 131094)
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>