staging: rtl8192u: Fix sparse warnings in r8192U_dm.c

Fix these warnings:
drivers/staging//rtl8192u/r8192U_dm.c:2307:49: warning: cast from restricted __le16
drivers/staging//rtl8192u/r8192U_dm.c:2308:44: warning: cast from restricted __le16
drivers/staging//rtl8192u/r8192U_dm.c:2309:44: warning: cast from restricted __le16

Signed-off-by: Matthieu Simon <gmatthsim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>