ztxdone is jumped to even if tty is NULL and tty_wakeup placed after
this label doesn't expect NULLed parameter, so this will cause an oops
in some situations (why they scheduled a wakeup there before remove
bottom half processing patch?).

wakeup only in the case when we have non-null tty struct.

Spotted by Adrian Bunk.
Signed-off-by: NJiri Slaby <jirislaby@gmail.com>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>