The reference to the upper USBDevice class is passed on
the EPHandler constructor.

this is the proper type to encapsulate a byte when data is available
or -1 when there is no data.

Previous 0 was always returned.

Only enabled if PIN_LED_TXL or PIN_LED_RXL is defined

Resolve compiler warning from performing arithmetic on void pointers

To be compliant with the one sent by the AVR core

The shortcut has some issues:

- sometimes it fails when sending an odd number of bytes (may be
  due to memory alignment?)
- the data pointer should point to "stable" data (and this is not
  guaranteed by caller, it may be some sort of temporary buffer)
- the SRAM is not guaranteed to start at 0x20000000

All the above problems must be properly fixed before reenabling
this part

* fix in_endpoint buffer size (was too big)
* use the same EP for CDC_ENDPOINT_OUT and CDC_ENDPOINT_IN (different buffers)
* fix CDC_ENDPOINT_IN ep type