serial: 8250: Make symbol 'brcmuart_debugfs_root' static

symbol 'brcmuart_debugfs_root' is not used outside of 8250_bcm7271.c,
so this commit marks it static.
Signed-off-by: Zucheng Zheng <zhengzucheng@huawei.com>
Link: https://lore.kernel.org/r/20210401074919.56573-1-zhengzucheng@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_bcm7271.c

@@ -239,7 +239,7 @@ struct brcmuart_priv {
 	u32		rx_abort;
 };
 
-struct dentry *brcmuart_debugfs_root;
+static struct dentry *brcmuart_debugfs_root;
 
 /*
  * Register access routines