net: bcmgenet: Delay PHY initialization to bcmgenet_open()

We are currently doing a full PHY initialization and even starting the
pHY state machine during bcmgenet_mii_init() which is executed in the
driver's probe function. This is convenient to determine whether we can
attach to a proper PHY device but comes at the expense of spending up to
10ms per MDIO transactions (to reach the waitqueue timeout), which slows
things down.

This also creates a sitaution where we end-up attaching twice to the
PHY, which is not quite correct either.

Fix this by moving bcmgenet_mii_probe() into bcmgenet_open() and update
its error path accordingly.

Avoid printing the message "attached PHY at address 1 [...]" every time
we bring up/down the interface and remove this print since it duplicates
what the PHY driver already does for us.

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>