crypto: stm32 - Simplify finup

The current finup code is unnecessarily convoluted.  There is no
need to call update and final separately as update already does
all the necessary work on its own.

Simplify this by utilising the HASH_FLAGS_FINUP bit in rctx to
indicate only finup and use the HASH_FLAGS_FINAL bit instead to
signify processing common to both final and finup.

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>