[netdrvr] gianfar: Determine TBIPA value dynamically

TBIPA needs to be set to a value (on connected MDIO buses) that
doesn't conflict with PHYs on the bus.  By hardcoding it to 0x1f,
we were preventing boards with PHYs at 0x1f from working properly.
Instead, scan the bus when it comes up, and find an address that
doesn't have a PHY on it.  The TBI PHY configuration code then
trusts that the value in TBIPA is either safe, or doesn't matter
(ie - it's not an active bus with other PHYs).

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>