ath11k: checking for NULL vs IS_ERR()

The ath11k_ce_alloc_ring() function returns error pointers on error, not
NULL.  The rest of the driver assumes that "pipe->src_ring" is either
valid or NULL so this patch introduces a temporary varaible to avoid
leaving it as an error pointer.

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>