net: phy: bcm7xxx: remove suspend callback for 28nm PHYs

BCM7xxx internal Gigabit PHY on 28nm process do not need anything
special to be done during suspend, remove the suspend callback since it
might be harmful rather than useful. While at it, update the comment
above bcm7xxx_suspend() to reflect that it applies only to 40nm and 65nm
process PHY devices.

Fixes: b560a58c ("net: phy: add Broadcom BCM7xxx internal PHY driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@greenl8ke.davemloft.net>