sh_eth: add sh_eth_cpu_data::cexcr flag

GEther controllers have CERCR/CEECR instead of CNDCR on the others.
Currently we are calling sh_eth_is_gether() in order to check for this,
however it would be simpler  to check the new 'cexcr' bitfield in the
'struct sh_eth_cpu_data';  then we'd be able to remove sh_eth_is_gether()
as there would be no callers left...

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