usb: dwc3: Set core clk rate to default upon device remove notification

commit 1eec2157d84dcb ("usb: dwc3: Update core clock rate based on
USB port speed.") only sets clk rate for HS/FS/LS device add notification.
In case super speed device is plugged in after LS/FS/HS device unplug in
absence of runtime suspend, core clk rate remains at high speed core clk
rate. Fix this issue by setting the core clk rate back to default when
a device is being removed from root hub.

Change-Id: I0ac37b7ff08a9a297441304eb2cfaff398d44bb9
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>