Fix DLC=0 IRQ handling

[caternuson]

Possible simple fix for #4.

Also fixes the onreceive example which was missing some config pin setup.

This was tested using two Feather M4 CAN's.

This was the DLC=0 TX test sketch:

#include <CANSAME5x.h>

CANSAME5x CAN;

void setup() {
  Serial.begin(115200);
  while (!Serial) delay(10);

  Serial.println("CAN Sender with DLC=0 packets");

  pinMode(PIN_CAN_STANDBY, OUTPUT);
  digitalWrite(PIN_CAN_STANDBY, false); // turn off STANDBY
  pinMode(PIN_CAN_BOOSTEN, OUTPUT);
  digitalWrite(PIN_CAN_BOOSTEN, true); // turn on booster

  // start the CAN bus at 250 kbps
  if (!CAN.begin(250000)) {
    Serial.println("Starting CAN failed!");
    while (1) delay(10);
  }
  Serial.println("Starting CAN!");
}

void loop() {
  // send packet: id is 11 bits, packet can contain up to 8 bytes of data
  Serial.print("Sending packet ... ");

  CAN.beginPacket(0x12);
  CAN.endPacket();

  Serial.println("done");

  delay(1000);
}

The RX test sketch was the feather_m4can_onreceive example, with the mods from this PR.
RX BEFORE

RX AFTER