sfc: Fix post-reset MAC selection

Modify falcon_switch_mac() to always set NIC_STAT_REG, even if the the
MAC is the same as it was before.  This ensures that the value is
correct after an online reset.

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