net: xilinx: axi_emac: Fix endless loop when no PHYs are connected

The index used to iterate over the possible PHYs in axiemac_phy_init was an
unsigned int and decremented. Therefor it was always >= 0 and never exited
the loop.

Signed-off-by: Patrick van Gelder <patrick.vangelder@nl.bosch.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>