USB: chipidea: fix use after free bug

The pointer to a platform_device struct must not be dereferenced after
the device has been unregistered.

This bug produces a crash when unloading the ci13xxx kernel module
compiled with CONFIG_PAGE_POISONING enabled.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Cc: stable <stable@vger.kernel.org> # 3.6
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>