From: Paul Mackerras <paulus@samba.org>

Since there are serial drivers (particularly the USB serial driver) that
call the line discipline receive_buf and write_wakeup routines at hard
interrupt level, I have changed the ppp_async code to cope with that.  It
now uses a tasklet so that it calls the generic PPP code at soft interrupt
level even if its receive_buf and write_wakeup entries are called at hard
interrupt level.

This patch has been lightly tested here with a keyspan USB serial adaptor
and also with the built-in modem on my tibook, which uses the pmac_zilog
driver (which hooks into the drivers/serial infrastructure).