ath10k: mark PM functions as __maybe_unused

When CONFIG_PM_SLEEP is disabled, we get a compile-time
warning:

drivers/net/wireless/ath/ath10k/pci.c:3417:12: error: 'ath10k_pci_pm_resume' defined but not used [-Werror=unused-function]
 static int ath10k_pci_pm_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ath/ath10k/pci.c:3401:12: error: 'ath10k_pci_pm_suspend' defined but not used [-Werror=unused-function]
 static int ath10k_pci_pm_suspend(struct device *dev)

Rather than fixing the #ifdef, this just marks both functions
as __maybe_unused, which is a more robust way to do this.

Fixes: 32faa3f0

 ("ath10k: add the PCI PM core suspend/resume ops")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>