[PATCH] rocket.c: Fix ldisc ref count handling

If bailing out because there is nothing to receive in rp_do_receive(),
tty_ldisc_deref is not called.  Failure to do so increases the ref count
and causes release_dev() to hang since it can't get the ref count to 0.

Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>