i2c: xgene-slimpro: dma_mapping_error() doesn't return an error code

The dma_mapping_error() function returns true if there is an error, it
doesn't return an error code.  We should return -ENOMEM.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org