Don't directly access key_bitlen

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/pkcs12.c b/library/pkcs12.c
index ce2dcf2..a0966f6 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -151,7 +151,7 @@
         return MBEDTLS_ERR_PKCS12_FEATURE_UNAVAILABLE;
     }
 
-    keylen = cipher_info->key_bitlen / 8;
+    keylen = mbedtls_cipher_info_get_key_bitlen(cipher_info) / 8;
 
     if ((ret = pkcs12_pbe_derive_key_iv(pbe_params, md_type, pwd, pwdlen,
                                         key, keylen,