Merge pull request #223 from ARMmbed/dev/Patater/fix-unused-mac_setup

psa: Unused key_bits is OK
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 291dcdb..d100eb1 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1869,6 +1869,7 @@
     else
 #endif /* MBEDTLS_MD_C */
     {
+        (void) key_bits;
         status = PSA_ERROR_NOT_SUPPORTED;
     }