USB: imx21-hcd: Fix isochronous endpoint idle

Release the hardware resources and reset the internal HCD state
associated with an isochronous endpoint when the last URB queued
for it completes.

Previously this was only done in then endpoint_disable() method
causing usbtest 15 and 16 to hang when run twice in succession
without a disconnect.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>