CONFIG_PM is defined when CONFIG_PM_SLEEP or CONFIG_PM_RUNTIME is defined,
however suspend and resume methods are only valid in the context of
CONFIG_PM_SLEEP. If only CONFIG_PM_RUNTIME is defined we get the following
warning (courtesy of Geerts randconfig builds):

	ad7879-i2c.c: warning: 'ad7879_i2c_resume' defined but not used
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>