mtd: gpmi: fix raw_buffer pointer double free issue

fix the raw_buffer pointer double free issue found by coverify.

CID 18344 (#2 of 2): Double free (USE_AFTER_FREE)
3. double_free: Calling gpmi_alloc_dma_buffer frees pointer
this->raw_buffer which has already been freed

Signed-off-by: Han Xu <han.xu@nxp.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>