Benchmark ECDSA: use digest size instead of key size

[SparkiDev]

Description

The key size can be larger than the maximum digest size supported by the sign and verify APIs. Calculate a reasonable digest size for the key size and bound it on the maximum digest size.

Fixes #9492

Testing

./configure --disable-shared CFLAGS=-DHAVE_ECC521 ./wolfcrypt/benchmark/benchmark -p521 -ecc

Checklist

• [ ] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation