sfc: Ignore parity errors in the other port's SRAM

Siena has a separate SRAM bank for each port.  On single-port boards
these can be merged together, so each port has an interrupt flag for
parity errors in the other port's SRAM.  Currently we do not enable
such merging and should mask this interrupt source.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>