crypto: simplify diffiehellman getFormat function

This commit aims to simplify the getFormat function in
diffiehellman.js.

PR-URL: #20246
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/crypto/diffiehellman.js

@@ -219,21 +219,15 @@ function encode(buffer, encoding) {
 }
 
 function getFormat(format) {
-  let f;
   if (format) {
     if (format === 'compressed')
-      f = POINT_CONVERSION_COMPRESSED;
-    else if (format === 'hybrid')
-      f = POINT_CONVERSION_HYBRID;
-    // Default
-    else if (format === 'uncompressed')
-      f = POINT_CONVERSION_UNCOMPRESSED;
-    else
+      return POINT_CONVERSION_COMPRESSED;
+    if (format === 'hybrid')
+      return POINT_CONVERSION_HYBRID;
+    if (format !== 'uncompressed')
       throw new ERR_CRYPTO_ECDH_INVALID_FORMAT(format);
-  } else {
-    f = POINT_CONVERSION_UNCOMPRESSED;
   }
-  return f;
+  return POINT_CONVERSION_UNCOMPRESSED;
 }
 
 module.exports = {