tty: n_gsm: Fix waking up upper tty layer when room available

Warn the upper layer when n_gms is ready to receive data
again. Without this the associated virtual tty remains blocked
indefinitely.

Fixes: e1eaea46

 ("tty: n_gsm line discipline")
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20200512115323.1447922-4-gregory.clement@bootlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>