ath10k: sanitity check for ep connectivity

Function ep_rx_complete is being called without NULL checking
in ath10k_htc_rx_completion_handler. Without such check, mal-
formed packet is able to cause jump to NULL.

ep->service_id seems a good candidate for sanity check as it is
used in usb.c.

Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200622022055.16028-1-bruceshenzk@gmail.com