r6040: move down second PHY detection to r6040_init_one

Instead of fully initializing an unusable second r6040
device, perform the PHY detection earlier and bail out
in r6040_init_one when we could not read the PHY identifier.

Signed-off-by: Mark Kelly <mark@bifferos.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>