ath11k: return -ENOMEM on allocation failure

Returning PTR_ERR(NULL) means success, but we should return -ENOMEM.

Fixes: 1399fb87

 ("ath11k: register MHI controller device for QCA6390")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200826113417.GE393664@mwanda