staging: rtl8723bs: force driver to be built as a module.

Due to the ton of global symbols, the driver has to be a module
otherwise the build blows up if it is compiled into the kernel.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723bs/Kconfig

 config RTL8723BS
 	tristate "Realtek RTL8723BS SDIO Wireless LAN NIC driver"
 	depends on WLAN && MMC && CFG80211
+	depends on m
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	---help---