cros_ec: Fix an error code is cros_ec_get_sku_id()

The ec_command_inptr() function returns negative error codes or
the number of bytes that it was able to read.  The cros_ec_get_sku_id()
function should return negative error codes.  Right now it returns
positive error codes or negative byte counts.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>