net: phy: mdio-bcm-unimac: factor busy polling loop

Factor the code that does the busy polling on the MDIO_BUSY bit since we
will have different code-paths for for completion depending on whether
we are using interrupts or polling.

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