sh_eth: define/use EESR_RX_CHECK macro

sh_eth_interrupt() uses the same Rx interrupt mask twice to check the interrupt
status register -- #define EESR_RX_CHECK  and use it instead.

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