sfp: improve support for direct-attach copper cables

Improve the support for direct-attach copper so that we avoid kernel
warning messages, and report the appropriate PORT_DA type to userspace.
Direct Attach cables can use a number of protocols depending on their
range of speeds.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>