sh_eth: handle only enabled E-MAC interrupts

The driver should only handle the enabled E-MAC interrupts, like it does
for the E-DMAC interrupts since commit 3893b273 ("sh_eth: workaround
for spurious ECI interrupt"),  so mask ECSR with  ECSIPR when reading it
in sh_eth_error().

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