Staging: rtl8192u: Added #include <linux/string.h> instead of <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>