ta: crypto_perf: correct coding style issue in symm tests

Sort out some coding style issue that are better being fixed for
test TAs implementation consistency. No functional changes.

Fixes: e18381f5 ("xtest: combine aes_perf and hash_perf TAs into crypto_perf")
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>