ANDROID: rfkill: fix unused function warning

As reported by kernelci, we get a harmless warning in this driver
when CONFIG_PM is disabled:

net/rfkill/core.c:810:12: warning: 'rfkill_resume' defined but not used [-Wunused-function]
net/rfkill/core.c:800:12: warning: 'rfkill_suspend' defined but not used [-Wunused-function]

This marks the functions as __maybe_unused to remove the #ifdef, and uses
a conditional reference to the pm operations instead, which will work
in any combination.

The patch is needed for both android-common-4.9 and 4.4.

Link: https://kernelci.org/build/id/59117c2f59b5147b06b12d54/logs/


Fixes: de6f7210 ("ANDROID: rfkill: Introduce CONFIG_RFKILL_PM and use instead of CONFIG_PM to power down")
Cc: Nick Pelly <npelly@google.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>