Receive may freeze if CAN saturated

If either the CANBUS is saturated or the MCU is busy for some time, both RX buffers of the MCP2515 may be in use. If the MCU does not catch up in processing the incoming messages, the second message received just adds another reason for the IRQ.
Thus reading a single message does not clear the IRQ conditon of the MCP2515 leading to a permanent lock up of the receiver program.

Signed-off-by: Ralf Edmund Stranzenbach <ralf@reswi.de>