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

drivers/staging/speakup/serialio.h

@@ -4,7 +4,7 @@
 #include <linux/serial.h>	/* for rs_table, serial constants */
 #include <linux/serial_reg.h>	/* for more serial constants */
 #ifndef __sparc__
-#include <asm/serial.h>
+#include <linux/serial.h>
 #endif
 
 /*