ata: ahci_brcmstb: Fix warnings with CONFIG_PM_SLEEP=n

When CONFIG_PM_SLEEP is disabled, brcm_ahci_{suspend,resume} are not
used, which causes such a build warning to occur:

  CC      drivers/ata/ahci_brcmstb.o
drivers/ata/ahci_brcmstb.c:212:12: warning: 'brcm_ahci_suspend' defined
but not used [-Wunused-function]
 static int brcm_ahci_suspend(struct device *dev)
            ^
drivers/ata/ahci_brcmstb.c:224:12: warning: 'brcm_ahci_resume' defined
but not used [-Wunused-function]
 static int brcm_ahci_resume(struct device *dev)
            ^
  LD      drivers/ata/built-in.o

Fixes: 766a2d97 ("ata: add Broadcom AHCI SATA3 driver for STB chips")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>