net: dsa: bcm_sf2: Introduce helper for port override offset

Depending upon the generation of switches, we have different offsets for
configuring a given port's status override where link parameters are
applied. Introduce a helper function that we re-use throughout the code
in order to let phylink callbacks configure the IMP/CPU port(s) in
subsequent changes.

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