Input: synaptics_i2c - wrap suspend and resume in CONFIG_PM_SLEEP

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

	synaptics_i2c.c: warning: 'synaptics_i2c_resume' defined but not used

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>