ath10k: cleanup calling ath10k_htt_rx_h_unchain

'ath10k_htt_rx_h_unchain' needs to be called only if the return
value from 'ath10k_htt_rx_amsdu_pop' is 1('chained msdu's'), this
change makes it more explicit and avoids doing a skb_peek, fetching
rx descriptor pointer, checking rx msdu decap format for the case of
ret = 0 (unchained msdus). Found this change during code walk through,
not sure if this addresses any issue.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>