Staging: rtl8192u: Replace header files

Files <asm/string.h> and <asm/errno.h> should not be included directly.
This patch fixes the following checkpatch.pl warnings:
Use #include <linux/errno.h> instead of <asm/errno.h>
Use #include <linux/string.h> instead of <asm/string.h>

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>