Fix DMA buffer size

The calculation of DMA buffer size is wrong, it should be this:

#define LED_DMA_BUFFER_SIZE ((LED_MAX_NBER_LEDS * sizeof(uint16_t) * (24/4)))+1

The current code uses 16 bytes instead of 2 bytes, making the DMA buffer quite large which slows down the refresh rate of the LEDs by a lot. 
