samples: usb: dfu: exclude lpcxpresso55s69_cpu0

Exclude lpcxpresso55s69_cpu0 from the USB DFU tests, this triggers a
build assert due to unsupported flash write block size (512 bytes,
mcuboot supports 8 to 32).

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
1 file changed