net: pcs: lynx: consolidate sgmii and 1000base-x config code

Consolidate lynx_pcs_config_1000basex() and lynx_pcs_config_sgmii() into
a single function. The differences between these two are:

- The value that the link timer is set to.
- The value of the IF_MODE register.

Everything else is identical.

This patch depends on "net: phylink: add QSGMII support to
phylink_mii_c22_pcs_encode_advertisement()".

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>