USB: gadget: f_uac1.c: Fix bug in accessing freed buffer pointer

Current driver is accessing the buffer pointer after it is freed.
This results in target crash. Hence fix the bug by setting pointer
variable to NULL before it is freed.

CRs-Fixed: 564289
Change-Id: I4df7513748f4378f6adce682102c8ddd3811ead8
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>