net: dsa: lan9303: Only allocate 3 ports

Save 2628 bytes on arm eabi by allocate only the required 3 ports.

Now that ds->num_ports is correct: In net/dsa/tag_lan9303.c
eliminate duplicate LAN9303_MAX_PORTS, use ds->num_ports.
(Matching the pattern of other net/dsa/tag_xxx.c files.)

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>