usb: dwc3: add CONFIG_PM_SLEEP to suspend/resume functions

Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following
build warning when CONFIG_PM_SLEEP is not selected. This is because
sleep PM callbacks defined by SET_SYSTEM_SLEEP_PM_OPS are only used
when the CONFIG_PM_SLEEP is enabled. Unnecessary CONFIG_PM ifdefs
are removed.

drivers/usb/dwc3/core.c:682:12: warning: 'dwc3_suspend' defined but not used [-Wunused-function]
drivers/usb/dwc3/core.c:709:12: warning: 'dwc3_resume' defined but not used [-Wunused-function]
drivers/usb/dwc3/dwc3-omap.c:430:12: warning: 'dwc3_omap_suspend' defined but not used [-Wunused-function]
drivers/usb/dwc3/dwc3-omap.c:440:12: warning: 'dwc3_omap_resume' defined but not used [-Wunused-function]
drivers/usb/dwc3/dwc3-exynos.c:185:12: warning: 'dwc3_exynos_suspend' defined but not used [-Wunused-function]
drivers/usb/dwc3/dwc3-exynos.c:194:12: warning: 'dwc3_exynos_resume' defined but not used [-Wunused-function]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>