ath11k: Fix error code in ath11k_core_suspend()

The "if (!ret)" condition is inverted and it should be "if (ret)".  It means
that we return success when we had intended to return an error code. This also
caused a spurious warning even when the suspend was successful:

[  297.186612] ath11k_pci 0000:06:00.0: failed to suspend hif: 0

Fixes: d1b0c338

 ("ath11k: implement suspend for QCA6390 PCI devices")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/X9nF17L2/EKOSbn/@mwanda