soundwire: depend on ACPI

The device cannot be probed on !ACPI and gives this warning:

drivers/soundwire/slave.c:16:12: warning: ‘sdw_slave_add’ defined but
not used [-Wunused-function]
 static int sdw_slave_add(struct sdw_bus *bus,
            ^~~~~~~~~~~~~

Cc: stable@vger.kernel.org
Fixes: 7c3cd189 ("soundwire: Add Master registration")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Link: https://lore.kernel.org/r/bd685232ea511251eeb9554172f1524eabf9a46e.1570097621.git.msuchanek@suse.deSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/soundwire/Kconfig

@@ -5,6 +5,7 @@
 
 menuconfig SOUNDWIRE
 	tristate "SoundWire support"
+	depends on ACPI
 	help
 	  SoundWire is a 2-Pin interface with data and clock line ratified
 	  by the MIPI Alliance. SoundWire is used for transporting data