ath11k: fix error return code in ath11k_dp_alloc()

Fix to return negative error code -ENOMEM from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: d0998eb8 ("ath11k: optimise ath11k_dp_tx_completion_handler")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200427104621.23752-1-weiyongjun1@huawei.com