misc: fastrpc: prevent memory leak in fastrpc_dma_buf_attach

In fastrpc_dma_buf_attach if dma_get_sgtable fails the allocated memory
for a should be released.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Link: https://lore.kernel.org/r/20190925152742.16258-1-navid.emamdoost@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
