USB: gadget: Fix memory leak in UASP function

The write buffer is allocated twice in bot_send_write_request
function.  Fix this by removing additional memory allocation.

Change-Id: I3253c149c3703621be30d0f84499d6af0aff8bc8
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>