phy: fix crash in fixed_phy_add()

Since e7f4dc35 ("mdio: Move allocation of interrupts into core"),
platforms which call fixed_phy_add() before fixed_mdio_bus_init() is
called (for example, because the platform code and the fixed_phy driver
use the same initcall level) crash in fixed_phy_add() since the
->mii_bus is not allocated.

Also since e7f4dc35, these interrupts are initalized to polling by
default.  The few (old) platforms which directly use fixed_phy_add()
from their platform code all pass PHY_POLL for the irq argument, so we
can keep these platforms not crashing by simply not attempting to set
the irq if PHY_POLL is passed.

Also, even if problems have not been reported on more modern platforms
which used fixed_phy_register() from drivers' probe functions, we return
-EPROBE_DEFER if the MDIO bus is not yet registered so that the probe is
retried later.

Fixes: e7f4dc35

 ("mdio: Move allocation of interrupts into core")
Signed-off-by: Rabin Vincent <rabinv@axis.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>