samples: siwx91x_ota: fix build error to align with latest mbedtls

Update the project configuration (mbedTLS options in prj.conf) so the
sample builds with current Zephyr and satisfies mbedtls/check_config.h when
RSA key exchange is enabled.

Disable MBEDTLS_USE_PSA_CRYPTO for TLS so legacy AES/GCM paths provide
MBEDTLS_SSL_HAVE_*; with PSA-for-TLS and no PSA AES wants, the client could
advertise no usable cipher suites.

Enable cipher and hash sets plus RSA key exchange with the required MD_C,
RSA_C, PKCS1, and ASN1 parse options.

Use sl_wifi_firmware_header_t for firmware header size instead of the
deprecated sl_si91x_firmware_header_t (WiseConnect 4.0).

Signed-off-by: Swamidas Nampalli <Swami.Das@silabs.com>

Author: silabs-SwamidasN

samples/siwx91x_ota/prj.conf

@@ -30,6 +30,14 @@ CONFIG_MBEDTLS_SSL_MAX_CONTENT_LEN=4096
 CONFIG_MBEDTLS_TLS_VERSION_1_2=y
 CONFIG_MBEDTLS_CIPHER_ALL_ENABLED=y
 CONFIG_MBEDTLS_HASH_ALL_ENABLED=y
+CONFIG_MBEDTLS_USE_PSA_CRYPTO=n
+CONFIG_PSA_WANT_KEY_TYPE_RSA_KEY_PAIR_IMPORT=y
+CONFIG_MBEDTLS_MD_C=y
+CONFIG_MBEDTLS_RSA_C=y
+CONFIG_MBEDTLS_PKCS1_V15=y
+CONFIG_MBEDTLS_PKCS1_V21=y
+CONFIG_MBEDTLS_ASN1_PARSE_C=y
+CONFIG_MBEDTLS_KEY_EXCHANGE_RSA_ENABLED=y
 
 CONFIG_ASSERT=y
 

samples/siwx91x_ota/src/main.c

@@ -421,7 +421,7 @@ static int ota_load_firmware(struct app_ctx *ctx)
 	__ASSERT(ctx->flash_buffer_len > 0, "No firmware data to process");
 
 	if (ctx->range_start == 0) {
-		buf_offset = sizeof(sl_si91x_firmware_header_t);
+		buf_offset = sizeof(sl_wifi_firmware_header_t);
 		status = sl_wifi_get_firmware_size(ctx->flash_buffer, &ota_calc_image_size);
 		if (status != SL_STATUS_OK) {
 			printf("Unable to fetch firmware size. Status: 0x%08x\n", status);