Staging: rtl8192u: Replace #include <asm/string.h>

The following patch fixes the checkpatch.pl warning:
WARNING: Use #include <linux/string.h> instead of #include <asm/string.h>
Signed-off-by: Dilek Uzulmez <dilekuzulmez@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192u/ieee80211/ieee80211_crypt_wep.c

@@ -14,7 +14,7 @@
 #include <linux/slab.h>
 #include <linux/random.h>
 #include <linux/skbuff.h>
-#include <asm/string.h>
+#include <linux/string.h>
 
 #include "ieee80211.h"