net: dsa: realtek: rtl8365mb: fix trap_door > 7

Trap door number is a 4-bit number divided in two regions (3 and 1-bit).
Both values were not masked properly. This bug does not affect supported
devices as they use up to port 7 (ext2). It would only be a problem if
the driver becomes compatible with 10-port switches like RTL8370MB and
RTL8310SR.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>