sfc: Rework MAC, PHY and board event handling

From: Steve Hodgson <shodgson@solarflare.com>

MAC, PHY and board events may be separately enabled and signalled.
Our current arrangement of chaining the polling functions can result
in events being missed.  Change them to be more independent.

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