net: pcs: xpcs: move searching ID list out of line

Move the searching of the physical ID out of xpcs_create() and into
its own xpcs_identify() function, which makes it self contained.
This reduces the complexity in xpcs_craete(), making it easier to
follow, rather than having a lot of once-run code in the big for()
loop.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>