mailbox: bcm-ferxrm-mailbox: Fix error check for dma_map_sg

[ Upstream commit 6b207ce8a96a71e966831e3a13c38143ba9a73c1 ]

dma_map_sg return 0 on error, fix the error check, and return -EIO
to caller.

Fixes: dbc049ee ("mailbox: Add driver for Broadcom FlexRM ring manager")
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>