Fix stm32.c type warnings

[dgarske]

Description

Fix stm32.c type warnings

Testing

wolfBoot with stm32wb-pka-1mb.config reports:

/home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.c
/home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.c: In function 'stm32_ecc_verify_hash_ex':
/home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.c:1037:22: error: comparison of integer expressions of different signedness: 'word32' {aka 'unsigned int'} and 'int' [-Werror=sign-compare]
 1037 |     else if (hashlen > size) {
      |                      ^
/home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.c: In function 'stm32_ecc_sign_hash_ex':
/home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.c:1144:22: error: comparison of integer expressions of different signedness: 'word32' {aka 'unsigned int'} and 'int' [-Werror=sign-compare]
 1144 |     else if (hashlen > size) {
      |                      ^
cc1: all warnings being treated as errors
make: *** [Makefile:554: /home/davidgarske/GitHub/wolfboot/lib/wolfssl/wolfcrypt/src/port/st/stm32.o] Error 1

Checklist

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