net: phy: reword get_phy_device() kerneldoc

Reword the get_phy_device() kerneldoc to be more explicit about how
we probe for the PHY, and what the various return conditions signify.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>