wil6210: fix build warnings without CONFIG_PM

The #ifdef checks are hard to get right, in this case some functions
should have been left inside a CONFIG_PM_SLEEP check as seen by this
message:

drivers/net/wireless/ath/wil6210/pcie_bus.c:489:12: error: 'wil6210_pm_resume' defined but not used [-Werror=unused-function]
drivers/net/wireless/ath/wil6210/pcie_bus.c:484:12: error: 'wil6210_pm_suspend' defined but not used [-Werror=unused-function]

Using an __maybe_unused is easier here, so I'm replacing all the
other #ifdef in this file as well for consistency.

Fixes: 94162666

 ("wil6210: run-time PM when interface down")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>