can, driver/mcp2512: shows strange frame reordering

Description

sending a group of multiple can frames shows a strange packet reordering issue

Steps to reproduce the issue

static int _send_fast(int argc, char **argv)
{
    (void) argc; (void) argv;
    int ret = 0;

    struct can_frame f = {
        .can_id = 1,
        .can_dlc = 8,
        .data = {0xff,0xff,0xff,0xff,
                 0xff,0xff,0xff,0xff}
    };
    uint8_t k = 1;
    for( int i = 8; i ; i --){
        f.data[0] = i;
        for(int j = 1 ; j<8; j++) f.data[j]=k++;
        ret = candev->driver->send(candev, &f);
        if (ret < 0) {
            puts("Failed to send CAN-message!");
        }    else {
            DEBUG("sent using mailbox: %d\n", ret);
        }

    }
    return 0;
}
and 
static const shell_command_t shell_commands[] = {
...
    { "send_fast", "send more data", _send_fast },
...
};

to tests/drivers/candev/

$ CAN_DRIVER=MCP2515 BOARD=same54-xpro make flash

into term

send_fast

CFLAGS +="-DCANDEV_MCP2515_DEFAULT_BITRATE=250000"

Expected results

listening on the can bus candump can0 I should receive:

  can0  001   [8]  08 01 02 03 04 05 06 07
  can0  001   [8]  07 08 09 0A 0B 0C 0D 0E
  can0  001   [8]  06 0F 10 11 12 13 14 15
  can0  001   [8]  05 16 17 18 19 1A 1B 1C
  can0  001   [8]  04 1D 1E 1F 20 21 22 23
  can0  001   [8]  03 24 25 26 27 28 29 2A
  can0  001   [8]  02 2B 2C 2D 2E 2F 30 31
  can0  001   [8]  01 32 33 34 35 36 37 38
... 
and so on

Actual results

  can0  001   [8]  08 01 02 03 04 05 06 07
  can0  001   [8]  06 0F 10 11 12 13 14 15
  can0  001   [8]  07 08 09 0A 0B 0C 0D 0E
  can0  001   [8]  04 1D 1E 1F 20 21 22 23
  can0  001   [8]  02 2B 2C 2D 2E 2F 30 31
  can0  001   [8]  03 24 25 26 27 28 29 2A

packet 5 and 1 are missing "Failed to send CAN-message!"

packets 6 and 7 and 2 and 3 are mixed up

The issue is very timing dependent a simple puts in between each frame like puts("frame send") works around the problem ( may therefor not showup with faster data rates or slower mcu (same54 is used )

Versions

current riot/master

RIOT-OS / RIOT