isdn: avoid calling tty_ldisc_flush() in atomic context

Remove the call to tty_ldisc_flush() from the RESULT_NO_CARRIER
branch of isdn_tty_modem_result(), as already proposed in commit
00409bb0.
This avoids a "sleeping function called from invalid context" BUG
when the hardware driver calls the statcallb() callback with
command==ISDN_STAT_DHUP in atomic context, which in turn calls
isdn_tty_modem_result(RESULT_NO_CARRIER, ~), and from there,
tty_ldisc_flush() which may sleep.

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