The PHY lookup helper "find_phy_by_path" could not lookup the PHY name
for paths from multi-phy devices.

An example for such a path would be:
'1e140000.pcie/pci0000:00/0000:00:01.0/0000:02:00.0+1'

The integer after the plus (+) character determines the PHY index of the
specific device in relation to the PHY with the lowest index of the
device.

For example, if the device provides phy2 and phy3, the above path would
describe phy3. In case the device provides phy0 and phy1, it would
describe phy1.

Rewrite the "find_phy_by_path" function to support those paths as well
as regular device paths in a universal manner.

Signed-off-by: David Bauer <mail@david-bauer.net>