hso: remove setting of low_latency flag

This patch removes the setting of the low_latency flag. 
tty_flip_buffer_push() is occasionally being called in irq context, which 
causes a hang if the low_latency flag is set.
Removing the low_latency flag only seems to impact the flush to ldisc, 
which will now be put on a workqueue.

Signed-off-by: Filip Aben <f.aben@option.com>
Signed-off-by: David S. Miller <davem@davemloft.net>