diag: Add protection while allocating memory during usb write

Add spin lock protection while allocating memory in diag_usb_write()
 to avoid race condition while allocating/freeing up the memory.

Change-Id: I0f068f163f6363b991413fad3d1e52cfc9d6af08
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>