net: phy: move PHY software reset to genphy_soft_reset

As pointed out by Shaohui, this function is generic for 10/100/1000
PHYs, but 10G PHYs might have a slightly different reset sequence which
prevents most of them from using this function.

Move the BMCR_RESET based software resent sequence to
genphy_soft_reset() in preparation for allowing PHY drivers to implement
a soft_reset() callback.

Reported-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>