usb: chipidea: drop unused field "device" from ci13xxx_ep

It was used as a shorthand for gadget's device in request mapping/unmapping
code, but now it's not used any more.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>