remoteproc/ste: fix memory leak on shutdown

Fixes coherent memory leakage, caused by non-deallocated
firmware image chunk.

Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Tarnyagin <dmitry.tarnyagin@stericsson.com>
Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: Ido Yariv <ido@wizery.com>
[slightly edit subject and commit log]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>