usb: gadget: composite: Fix double free memory bug

configfs_dev_cleanup function can double free os_desc
and buffer when called from different context. For
example, this can be called from composite_unbind() and
when composite_bind() fails. Fix this issue by setting
request and buffer pointer to NULL after kfree.

CRs-Fixed: 1013316
Change-Id: I6e87289627b23fc368f990fc7962854eeb3fbbc1
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>