rtlwifi: rtl_usb: Fix missing entry in USB driver's private data

These drivers need to be able to reference "struct ieee80211_hw" from
the driver's private data, and vice versa. The USB driver failed to
store the address of ieee80211_hw in the private data. Although this
bug has been present for a long time, it was not exposed until
commit ba9f93f8 ("rtlwifi: Fix enter/exit power_save").

Fixes: ba9f93f8 ("rtlwifi: Fix enter/exit power_save")
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>