moreover, I was reading the hardware fifo both at interrupt level and
at tasklet level.  The tasklet read-out is mandatory if we are using
a timer instead of an interrupt (the timer is the tasklet function),
but should be avoided if we have interrupts.

Sometimes I got duplicate samples because the tasklet reading the
hardware fifo was preempted by the interrupt handler.

Now I can stamp 50kHz bursts withouth data loss, provided the zio
buffer is big enough.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>