staging: gdm724x: prevent module from being built in

There are global symbols here that conflict with the rest of the kernel,
preventing it from being built, so do not allow this module to be built
in.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Won Kang <wonkang@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gdm724x/Kconfig

@@ -4,7 +4,7 @@
 
 config LTE_GDM724X
 	tristate "GCT GDM724x LTE support"
-	depends on NET && USB
+	depends on NET && USB && m
 	help
 	  This driver supports GCT GDM724x LTE chip based USB modem devices.
 	  It exposes 4 network devices to be used per PDN and 2 tty devices to be