net: pcs: lynx: make lynx_pcs_create() static

We no longer need to export lynx_pcs_create() for drivers to use as we
now have all the functionality we need in the two new creation helpers.
Remove the export and prototype, and make it static.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>