net: bcmgenet: Fix early link interrupt enabling

Link interrupts are enabled in init_umac(), which is too early for us to
process them since we do not yet have a valid PHY device pointer. On
BCM7425 chips for instance, we will crash calling phy_mac_interrupt()
because phydev is NULL.

Fix this by moving the link interrupts enabling in
bcmgenet_netif_start(), under a specific function:
bcmgenet_link_intr_enable() and while at it, update the comments
surrounding the code.

Fixes: 6cc8e6d4

 ("net: bcmgenet: Delay PHY initialization to bcmgenet_open()")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>