fix desc_end in vendord_open()

[Cynventria]

minor fix on calculation of desc_end in vendord_open( ) for descriptor prasing

Describe the PR

fix desc_end calculation
In vendor device class, function vendord_open()

Additional context

In vendor_device.c, original code
https://github.com/hathach/tinyusb/blob/master/src/class/vendor/vendor_device.c

  const uint8_t* p_desc = tu_desc_next(desc_itf);
  const uint8_t* desc_end = p_desc + max_len;

desc_end is calaulated by p_desc + max_len which is u_desc_next(desc_itf) + max_len ,
but parameter max_len include length of desc_itf,
which make desc_end exceed the real end of descriptor by length of desc_itf

It won't cause issue since while loop stopped when all endpoints are founded
But error may occur when trying to peek descriptors after last endpoint

Since vendor_device is an ideal reference for developing new USB function
I suppose it'll be better to do the minor fix

  const uint8_t* p_desc = tu_desc_next(desc_itf);
  const uint8_t* desc_end = (uint8_t const*)desc_itf + max_len;

Encountered this when I try to check if next interface an AlternateSetting of current interface

thanks.

[HiFiPhile]

Thank you