drivers/net/dsa/Kconfig: Let NET_DSA_BCM_SF2 depend on HAS_IOMEM

NET_DSA_BCM_SF2 need HAS_IOMEM, so depend on it, the related error (with
allmodconfig under um):

    CC [M]  drivers/net/dsa/bcm_sf2.o
  drivers/net/dsa/bcm_sf2.c: In function ‘bcm_sf2_sw_setup’:
  drivers/net/dsa/bcm_sf2.c:487:3: error: implicit declaration of function ‘iounmap’ [-Werror=implicit-function-declaration]
     iounmap(*base);
     ^
Signed-off-by: NChen Gang <gang.chen.5i5j@gmail.com>
Acked-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/dsa/Kconfig

@@ -47,6 +47,7 @@ config NET_DSA_MV88E6171
 
 config NET_DSA_BCM_SF2
 	tristate "Broadcom Starfighter 2 Ethernet switch support"
+	depends on HAS_IOMEM
 	select NET_DSA
 	select NET_DSA_TAG_BRCM
 	select FIXED_PHY if NET_DSA_BCM_SF2=y