crypto: stm32 - Move polling into do_one_request

There is no need to poll separate for update and final.  We could
merge them into do_one_request.

Also fix the error handling so that we don't poll (and overwrite
the error) when an error has already occurred.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>