sh_eth: add/use RMCR.RNC bit

Declare 'enum RMCR_BIT' containing the single member for the RMCR.RNC bit and
replace bare numbers in the driver by  this mnemonic.

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