rtlwifi: rtl8821ae: Fix HW_VAR_NAV_UPPER operation

The cast here is wrong.  We want to cast the pointer but we accidentally
do a no-op cast of the value.  We normally want to set us_nav_upper to
WIFI_NAV_UPPER_US (30000) but because of this bug we instead set it to
184 on little endian systems and 0 on big endian ones.

Fixes: 3c05bedb ("Staging: rtl8812ae: Add Realtek 8821 PCI WIFI driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>