net: phy: document has_fixups field

has_fixups was introduced to help keeping track of fixups/quirks running
on a PHY device, but we did not update the comment above struct
phy_device accordingly.

Fixes: b0ae009f3dc14 (net: phy: add "has_fixups" boolean property")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>