Fix build of v3.6 with unset MBEDTLS_DHM_C but MBEDTLS_USE_PSA_CRYPTO set (fixes #9188)

Avoid compiler warning about size comparison (like in commit 7910cdd):

Clang builds fail, warning about comparing uint8_t to a size that may be >255.

Signed-off-by: Michael Schuster <michael@schuster.ms>
diff --git a/ChangeLog.d/fix-clang-psa-build-without-dhm.txt b/ChangeLog.d/fix-clang-psa-build-without-dhm.txt
new file mode 100644
index 0000000..7ae1c68
--- /dev/null
+++ b/ChangeLog.d/fix-clang-psa-build-without-dhm.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix Clang compilation error when MBEDTLS_USE_PSA_CRYPTO is enabled
+     but MBEDTLS_DHM_C is disabled. Reported by Michael Schuster in #9188.