wifi: ath11k: Fix NULL pointer check in ath11k_ce_rx_post_pipe() ERROR code

[Baichuan Qi]

thanks for your reply

> May the detection of a null pointer for the data structure members
> “dest_ring” or “status_ring” really be interpreted as a successful execution
> of the function “ath11k_ce_rx_post_pipe”?

i submit this patch just want to ensure the last codes after
`if(...)` in `ath11k_ce_rx_post_pipe` can be successfully executed.

There is no error handling for NULL dest_ring (or status_ring)
in other funtions calling `ath11k_ce_rx_post_pipe()`, because it 
would return 0, like successful end.

I think this patch can not change so many codes in other functions in
ce. This may involve a lot of error handling operations,
and depending on the severity of situation, the driver may even
re_setup the ring.

I will submit a v5 patch, and in that patch an error code will be
returned.

Baichuan Qi