rtlwifi: rtl8192cu: Add missing case in rtl92cu_get_hw_reg

Driver was reporting 'switch case not processed' after association,
so HW_VAR_KEEP_ALIVE was added and filled similarily to other drivers.

Positive side effect to this seems to be a bit more stable connection.

Signed-off-by: Priit Laes <plaes@plaes.org>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>