bas_gigaset: use tasklet_hi_schedule for timing critical tasklets

The tasklets for isochronous data transfer need to run within 8 msec
to avoid over/underruns, so schedule them with high priority to fix
reported issues with occasional over/underruns.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>