staging: rtl8192u: remove #ifdef RTL8192U guard in r8192U_dm.c

This patch removes #ifdef RTL8192U header guards from r8192U_dm.c, since
RTL8192U is defined in the included r8192U.h header.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>