net: dsa: bcm_sf2: Have PHYLINK configure CPU/IMP port(s)

Remove the artificial limitations imposed upon
bcm_sf2_sw_mac_link_{up,down} and allow us to override the link
parameters for IMP port(s) as well as regular ports by accounting for
the special differences that exist there.

Remove the code that did override the link parameters in
bcm_sf2_imp_setup().

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>