net: systemport: add NET_DSA dependency

The notifier cause a link error when NET_DSA is a loadable
module:

drivers/net/ethernet/broadcom/bcmsysport.o: In function `bcm_sysport_remove':
bcmsysport.c:(.text+0x1582): undefined reference to `unregister_dsa_notifier'
drivers/net/ethernet/broadcom/bcmsysport.o: In function `bcm_sysport_probe':
bcmsysport.c:(.text+0x278d): undefined reference to `register_dsa_notifier'

This adds a dependency that forces the systemport driver to be
a loadable module as well when that happens, but otherwise
allows it to be built normally when DSA is either built-in or
completely disabled.

Fixes: d1565763

 ("net: systemport: Establish lower/upper queue mapping")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>