fbnic: Replace use of internal PCS w/ Designware XPCS

fbnic: Replace use of internal PCS w/ Designware XPCS

As we have exposed the PCS registers via the SWMII we can now start looking
at connecting the XPCS driver to those registers and let it mange the PCS
instead of us doing it directly from the fbnic driver.

For now this just gets us the ability to detect link. The hope is in the
future to add some of the vendor specific registers to begin enabling XPCS
configuration of the interface.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Link: https://patch.msgid.link/176374325295.959489.14521115864034905277.stgit@ahduyck-xeon-server.home.arpa
Signed-off-by: Paolo Abeni <pabeni@redhat.com>