realtek: correct egress frame port verification

realtek: correct egress frame port verification

Destination switch ports for outgoing frame can range from 0 to
CPU_PORT-1.

Refactor the code to only generate egress frame CPU headers when a valid
destination port number is available, and make the code a bit more
consistent between different switch generations. Change the dest_port
argument's type to 'unsigned int', since only positive values are valid.

This fixes the issue where egress frames on switch port 0 did not
receive a VLAN tag, because they are sent out without a CPU header.
Also fixes a potential issue with invalid (negative) egress port numbers
on RTL93xx switches.

Reported-by: Arınç ÜNAL <arinc.unal@xeront.com>
Suggested-by: Birger Koblitz <mail@birger-koblitz.de>
Tested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
(cherry picked from commit 1773264a0c6da099af7f36046f95f0126d6de1eb)