Add a return value check when reading data from the FIFO register.

Cc: <stable@vger.kernel.org>
Reviewed-by: Jason Gunthorpe <jason.gunthorpe@obsidianresearch.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Reviewed-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>