rtlwifi: rtl8192cu: Fix variable isfirst_ampdu

rtl92cu_rx_query_desc set a isampdu twice.
but second code is related to isfirst_ampdu.
so i change it.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>