sh_eth: use ETH_ZLEN instead of home-grown #define

The driver #define's and uses ETHERSMALL macro for the minimum Ethernet frame
size for which we have a standard macro ETH_ZLEN.  Use the latter  instead of
the home-grown one.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>