sfc: Extend self-tests

Include PMA/PMD in loopback self-tests as intended.

Add NVRAM checksum validation and include it in self-tests.

Add register self-tests.

Run PHY self-tests where available.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>