spi: Correct two error return values

When an error number is provided we should use it, not change it. This fixes
the SPI and SPI flash tests.

One of these is long-standing. The other seems to have been introduced by
commit 1e90d9fd (sf: Move read_id code to sf_ops).

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 1e90d9fd (sf: Move read_id code to sf_ops)
Reviewed-by: Jagan Teki <jteki@openedev.com>
Tested-by: Jagan Teki <jteki@openedev.com>