tty: serial: mux: include <linux/io.h> instead of <asm/io.h>

Include the more general linux/io.h instead of asm/io.h
as checkpatch suggests.
Signed-off-by: Zihao Tang <tangzihao1@hisilicon.com>
Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Link: https://lore.kernel.org/r/1620874904-39285-6-git-send-email-f.fangjian@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mux.c

@@ -21,7 +21,7 @@
 #include <linux/console.h>
 #include <linux/delay.h> /* for udelay */
 #include <linux/device.h>
-#include <asm/io.h>
+#include <linux/io.h>
 #include <asm/irq.h>
 #include <asm/parisc-device.h>