net: sfp: Add helper to return the SFP bus name

Knowing the bus name is helpful when we want to expose the link topology
to userspace, add a helper to return the SFP bus name.

This call will always be made while holding the RTNL which ensures
that the SFP driver won't unbind from the device. The returned pointer
to the bus name will only be used while RTNL is held.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Suggested-by: "Russell King (Oracle)" <linux@armlinux.org.uk>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>