usb: misc: fix memory leaks

Replace calls to platform_device_del with platform_device_unregister
which calls device_put to properly free the device structure.

CRs-fixed: 366005
Change-Id: Ic3d82e50383fa475f50c73e5982ef718a9eaa26f
Signed-off-by: Jack Pham <jackp@codeaurora.org>