This makes `rx_buffer` and `tx_buffer` *pointers*
volatile in order to avoid caching them in a register.
The original notation meant "a pointer to a volatile value"
(equivalent of `volatile uint8_t *`). This resulted in
`while(rx_buffer[ep_num] != NULL) ;` loop to get stuck
forever, even though the IRQ handler set the `rx_buffer[ep_num] = NULL`.
