spi: bcm2835: Fix error return code in bcm2835_dma_init()

Fix to return negative error code -ENOMEM from the dma mapping error
handling case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200506125607.90952-1-weiyongjun1@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>