wlcore: fix unused function warning

The newly added wlcore_fw_sleep function is called conditionally,
which causes a warning without CONFIG_PM:

drivers/net/wireless/ti/wlcore/main.c:981:12: error: 'wlcore_fw_sleep' defined but not used [-Werror=unused-function]

Instead of trying to keep track of what should be in the #ifdef and what
should not, it's easier to mark the top-level suspend/resume functions
as __maybe_unused so the compiler can silently drop all the unused code.

Fixes: 37bf241b ("wlcore: allow elp during wowlan suspend")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>