ssb: add Kconfig entry for compiling SoC related code

This allows saving a little of space when not using ssb on Broadcom SoC.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/ssb/ssb_private.h

@@ -161,7 +161,9 @@ static inline int ssb_sdio_init(struct ssb_bus *bus)
  * host_soc.c
  **************************************************/
 
+#ifdef CONFIG_SSB_HOST_SOC
 extern const struct ssb_bus_ops ssb_host_soc_ops;
+#endif
 
 /* scan.c */
 extern const char *ssb_core_name(u16 coreid);