tty: serial_core: fix error code returned by uart_register_driver()

uart_register_driver() returned -ENOMEM on any error, even when
tty_register_driver() call returned another one, such as -EBUSY.

Signed-off-by: Sergey Organov <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/serial_core.c

···
 int uart_register_driver(struct uart_driver *drv)
 {
 	struct tty_driver *normal;
-	int i, retval;
+	int i, retval = -ENOMEM;
 
 	BUG_ON(drv->state);
 
···
 out_kfree:
 	kfree(drv->state);
 out:
-	return -ENOMEM;
+	return retval;
 }
 
 /**