staging: rtl8192e: use memdup_user to simplify code

Use memdup_user rather than duplicating its implementation. This patch
fixes the following coccinelle warnings:

drivers/staging/rtl8192e/rtl8192e/rtl_core.c:2598:8-15: WARNING opportunity for memdup_user
drivers/staging/rtl8192e/rtllib_softmac.c:3594:9-16: WARNING opportunity for memdup_user

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>