net: phy: fix a bug in get_phy_c45_ids

When probing devices-in-package for a c45 phy, device zero is the last
device to probe, however, if driver reads 0 from device zero,
c45_ids->devices_in_package is set to '0', the loop condition of probing
will be matched again, see codes below:

for (i = 1;i < num_ids && c45_ids->devices_in_package == 0;i++)

driver will run in a dead loop.

This patch restructures the bug and confusing loop, it provides a helper
function get_phy_c45_devs_in_pkg which to read devices-in-package registers
of a MMD, and rewrites the loop with using the helper function.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>