net: phy: at803x: add suspend/resume callbacks

When WOL is enabled, the chip can't be put into power-down (BMCR_PDOWN)
mode, as that will also switch off the MAC, which consequently leads to
a link loss.

Use BMCR_ISOLATE in that case, which will at least save us some
milliamperes in comparison to normal operation mode.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>