ata: libahci: Only warn for AHCI_HFLAG_MULTI_MSI set when genuine custom irq handler implemented

For an AHCI controller with AHCI_HFLAG_MULTI_MSI flag set, we may get the
following log, regardless of whether a custom irq handler was implemented
or not:

[   14.700238] ahci 0000:74:03.0: both AHCI_HFLAG_MULTI_MSI flag set and custom irq handler implemented

This is because we can set hpriv->irq_handler to
ahci_single_level_irq_intr() if not already set, in
ahci_init_one()->ahci_pci_save_initial_config()->ahci_save_initial_config().

Stop having this warn being misleading by adding a check for
hpriv->irq_handler != ahci_single_level_irq_intr.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>