net: pcs: rearrange C73 functions to prepare for C37 support later

The current implementation for XPCS is validated for C73, so we rename them
to have _c73 suffix and introduce a set of functions to use an_mode flag
to switch between C73 and C37 AN later.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>