Simplify unnecessarily complex error code handling

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/cipher.c b/library/cipher.c
index 73aaa5a..b398a60 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -915,7 +915,7 @@
         in_padding = mbedtls_ct_bool_and(in_padding, mbedtls_ct_bool_not(is_nonzero));
     }
 
-    return -((int) mbedtls_ct_uint_if_else_0(bad, -MBEDTLS_ERR_CIPHER_INVALID_PADDING));
+    return (int) mbedtls_ct_uint_if_else_0(bad, MBEDTLS_ERR_CIPHER_INVALID_PADDING);
 }
 #endif /* MBEDTLS_CIPHER_PADDING_ONE_AND_ZEROS */
 
@@ -962,7 +962,7 @@
         bad = mbedtls_ct_bool_or(bad, nonzero_pad_byte);
     }
 
-    return -((int) mbedtls_ct_uint_if_else_0(bad, -MBEDTLS_ERR_CIPHER_INVALID_PADDING));
+    return (int) mbedtls_ct_uint_if_else_0(bad, MBEDTLS_ERR_CIPHER_INVALID_PADDING);
 }
 #endif /* MBEDTLS_CIPHER_PADDING_ZEROS_AND_LEN */