staging: r8188eu: use proper way to build a module

It seems that for now, we can only build this driver as a module.

Use the same mechanism as other drivers (such as rtl8723bs or the
deprecated rtl8188eu) to enforce building as a module, i.e. depend on m
in Kconfig instead of setting CONFIG_R8188EU = m in the Makefile.

If we set CONFIG_R8188EU in the Makefile, this setting will not be visible
in .config.
Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210811201450.31366-4-martin@kaiser.cxSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/r8188eu/Kconfig

@@ -2,6 +2,7 @@
 config R8188EU
 	tristate "Realtek RTL8188EU Wireless LAN NIC driver"
 	depends on WLAN && USB && CFG80211
+	depends on m
 	select WIRELESS_EXT
 	select WEXT_PRIV
 	select LIB80211

drivers/staging/r8188eu/Makefile

@@ -4,8 +4,6 @@ EXTRA_CFLAGS += -O1
 
 ccflags-y += -D__CHECK_ENDIAN__
 
-CONFIG_R8188EU = m
-
 CONFIG_USB_HCI = y
 
 CONFIG_BT_COEXIST = n