Endpoint configuration
----------------------
0   Control endpoint
1   keyboard
2   mouse
3   debug
4   extra key(consumer/system)
5   nkro keyboard(supported only on ATmega32U4/16U4 and AT90USB64/128)


ATmega32U4/16U4, AT90USB64/128
• Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint
• Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode.
• Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode.

AT90USB82/162, ATmega8U2/16U2/32U2
• Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint
• Endpoints 1 and 2: programmable size FIFO up to 64 bytes.
• Endpoints 3 and 4: programmable size FIFO up to 64 bytes with ping-pong mode.

ping-pong mode means double buffer feature.


NOTE: ATmega8U2/16U2/32U2 is not supported with PJRC stack at this time.
TODO: Macro definition for ATmega8U2/16U2/32U2 in usb.h