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

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>