staging: speakup: Fixed warning <linux/serial.h> instead of <asm/serial.h>

This patch fixes warning: 'use <linux/serial.h> instead <asm/serial.h>'
found by checkpatch

Signed-off-by: Melike Yurtoglu <aysemelikeyurtoglu@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>