phy: Fix u-boot coruption when fixed-phy is used

When fixed-link phy is used subnode offset is used as phy address. This
number is bigger then space allocated for bus structure (allocated via
mdio_alloc).
bus->phymap[] array has PHY_MAX_ADDR size (32).
That's why writing bus->phymap[addr] where addr is < 0 or > PHY_MAX_ADDR
is causing write to memory which can caused full U-Boot crash.

The patch is checking if address is in correct range.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>