net: dsa: realtek: rtl8365mb: remove port_mask private data member

There is no real need for this variable: the line change interrupt mask
is sufficiently masked out when getting linkup_ind and linkdown_ind in
the interrupt handler.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>