staging: rtl8192u: use memdup_user to simplify code

Use memdup_user rather than duplicating its implementation. Fix the
following coccinelle warnings:

drivers/staging/rtl8192u/r8192U_core.c:3792:7-14: WARNING opportunity for memdup_user
drivers/staging/rtl8192u/ieee80211/ieee80211_softmac.c:3153:9-16: WARNING opportunity for memdup_user

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