sh_eth: fix RX buffer size calculation

The RX buffer size calulation failed to account for the length granularity
(which is now 32 bytes)...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit cb368595

)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>