drivers: staging: rtl8723au: fix "warning: cast to restricted __le16"

This patch fixes the sparse warning: "cast to restricted __le16" reported
for rtl8723au/hal/rtl8723au_xmit.c
Signed-off-by: NPiotr Witoslawski <pwitos@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8723au/hal/rtl8723au_xmit.c

@@ -42,7 +42,7 @@ static int urb_zero_packet_chk(struct rtw_adapter *padapter, int sz)
 
 static void rtl8192cu_cal_txdesc_chksum(struct tx_desc	*ptxdesc)
 {
-		u16	*usPtr = (u16 *)ptxdesc;
+		__le16	*usPtr = (__le16 *)ptxdesc;
 		u32 count = 16;		/*  (32 bytes / 2 bytes per XOR) => 16 times */
 		u32 index;
 		u16 checksum = 0;