kv: Some minor file cache_buffer tweaks

- Unconditionally pass buffer as cache_buffer in lfs3_set now that we
  rely on LFS3_o_WRSET

- Swapped true -> 1 for non-null don't-care buffer pointer

Saved one instruction as expected for the conditional assignment, but
added a bit of stack. Weird, but probably just compiler noise:

           code          stack          ctx
  before: 37836           2408          636
  after:  37832 (-0.0%)   2416 (+0.3%)  636 (+0.0%)

lfs3.c

@@ -13939,7 +13939,7 @@ int lfs3_file_ckdata(lfs3_t *lfs3, lfs3_file_t *file) {
 // kv file config, we need to explicitly disable the file cache
 static const struct lfs3_file_config lfs3_file_kvconfig = {
     // TODO is this the best way to do this?
-    .cache_buffer = (uint8_t*)true,
+    .cache_buffer = (uint8_t*)1,
     .cache_size = 0,
 };
 
@@ -13998,7 +13998,7 @@ int lfs3_set(lfs3_t *lfs3, const char *path,
     //   to try to commit small files atomically
     //
     struct lfs3_file_config cfg = {
-        .cache_buffer = (buffer) ? (uint8_t*)buffer : (uint8_t*)true,
+        .cache_buffer = (uint8_t*)buffer,
         .cache_size = size,
     };
     lfs3_file_t file;