sh_eth: Use bool as return type of sh_eth_is_gether()

Return a boolean from sh_eth_is_gether() and refactor it as a one-liner.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>