sfp: add sff module support

Add support for SFF modules, which are soldered down SFP modules.
These have a different phys_id value, and also have the present and
rate select signals omitted compared with their socketed counter-parts.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>