usb: dwc3: core: Power-off core/PHYs on system_suspend in host mode

Commit 689bf72c ("usb: dwc3: Don't reinitialize core during
host bus-suspend/resume") updated suspend/resume routines to not
power_off and reinit PHYs/core for host mode.
It broke platforms that rely on DWC3 core to power_off PHYs to
enter low power state on system suspend.

Perform dwc3_core_exit/init only during host mode system_suspend/
resume to addresses power regression from above mentioned patch
and also allow USB session to stay connected across
runtime_suspend/resume in host mode. While at it also replace
existing checks for HOST only dr_mode with current_dr_role to
have similar core driver behavior for both Host-only and DRD+Host
configurations.

Fixes: 689bf72c ("usb: dwc3: Don't reinitialize core during host bus-suspend/resume")
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>