ata: brcm: mark PM functions as __maybe_unused

The new shutdown callback causes a link failure:

drivers/ata/ahci_brcm.c: In function 'brcm_ahci_shutdown':
drivers/ata/ahci_brcm.c:552:8: error: implicit declaration of function 'brcm_ahci_suspend'; did you mean 'brcm_ahci_shutdown'? [-Werror=implicit-function-declaration]
  ret = brcm_ahci_suspend(&pdev->dev);
        ^~~~~~~~~~~~~~~~~

Remove the incorrect #ifdef and use __maybe_unused annotations
instead to make this more robust.

Fixes: 7de9b168 ("ata: ahci_brcm: Add a shutdown callback")
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>