realtek: don't treat first multicast portmask entry as reserved

realtek: don't treat first multicast portmask entry as reserved

There doesn't appear to be a reason to do this, as only the last entry
is actually reserved for unknown multicast.

This also fixes two issues:
 - As the increment happened after the bounds check, the value of the
   actually reserved last entry could be overwritten.
 - On deletion of entries, a corresponding decrement was missing,
   causing the wrong entry to be marked as free.

Fixes: cde31976e375 ("realtek: Add support for Layer 2 Multicast")
Signed-off-by: Jan Hoffmann <jan@3e8.eu>