tulip: cleanup by using ARRAY_SIZE()

In this situation then ARRAY_SIZE() and sizeof() are the same, but we're
really dealing with array indexes and not byte offsets so ARRAY_SIZE()
is cleaner.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>