fixed a compiling warning

warning:  #177-D: variable "i" was declared but never referenced

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2469 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/net/lwip/src/arch/sys_arch.c

@@ -534,7 +534,7 @@ u32_t sys_jiffies(void)
 
 u32_t sio_read(sio_fd_t fd, u8_t *buf, u32_t size)
 {
-    u32_t len, i;
+    u32_t len;
 
     RT_ASSERT(fd != RT_NULL);
 
@@ -547,7 +547,6 @@ u32_t sio_read(sio_fd_t fd, u8_t *buf, u32_t size)
 
 u32_t sio_write(sio_fd_t fd, u8_t *buf, u32_t size)
 {
-    u32_t i;
     RT_ASSERT(fd != RT_NULL);
 
     return rt_device_write((rt_device_t)fd, 0, buf, size);