spi: sirf: replace BUG condition by error code for unsupported transfer

return error for unsupported bits-per-word format, BUG() is not right
for this scenerios as we are not an ASSERT but an error handler.
Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <baohua.song@csr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-sirf.c

@@ -559,7 +559,8 @@ spi_sirfsoc_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
 		sspi->tx_word = spi_sirfsoc_tx_word_u32;
 		break;
 	default:
-		BUG();
+		dev_err(&spi->dev, "bpw %d not supported\n", bits_per_word);
+		return -EINVAL;
 	}
 
 	sspi->word_width = DIV_ROUND_UP(bits_per_word, 8);