usb: core: Allow secondary event ring clean upon disconnect

If device is disconnected in the middle of data transfer
there is a possibility of pending events in the secondary
event ring. As a result when xHC is halted as part of device
disconnect, device is not getting detected upon re-connect.
Fix the issue by acknowledging the pending events upon
device disconnect. Also this allows to finish the clean up
of event ring otherwise this results into a memory leak.

Change-Id: I2ec3d74867e0129dad395d1dbe920b468ac5949d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>