fix(lua): io.read always reading 256 bytes rather than specified value

[frankiearzu]

Fixes #5832

Summary of changes:
Use parameter "n" instead of the fix LUAL_BUFFERSIZE

[frankiearzu]

Validated in TX16S.. Forward Programming (DSMTools) works again in 2.11

[raphaelcoeffic]

@frankiearzu This code comes straight from Lua code. I believe the intent is actually to buffer I/O ops by reading full buffer size when possible, and then returning only what you asked. You might want to check the original Lua code to see how it is actually supposed to work. Reading a couple bytes from FatFs is very inefficient.

[raphaelcoeffic]

Nevermind, the original Lua code is doing the same:

static int read_chars (lua_State *L, FILE *f, size_t n) {
  size_t nr;  /* number of chars actually read */
  char *p;
  luaL_Buffer b;
  luaL_buffinit(L, &b);
  p = luaL_prepbuffsize(&b, n);  /* prepare buffer to read whole block */
  nr = fread(p, sizeof(char), n, f);  /* try to read 'n' chars */
  luaL_addsize(&b, nr);
  luaL_pushresult(&b);  /* close buffer */
  return (nr > 0);  /* true iff read something */
}