devtmpfs: simplify initialization of mount_dev

Avoid a bit of ifdeffery by using the IS_ENABLED() helper.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20200115184154.3492-4-linux@rasmusvillemoes.dkSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/devtmpfs.c

@@ -30,11 +30,7 @@
 
 static struct task_struct *thread;
 
-#if defined CONFIG_DEVTMPFS_MOUNT
-static int mount_dev = 1;
-#else
-static int mount_dev;
-#endif
+static int mount_dev = IS_ENABLED(CONFIG_DEVTMPFS_MOUNT);
 
 static DEFINE_SPINLOCK(req_lock);