Char: cyclades, fix potential NULL dereference

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: Jiri Slaby <jirislaby@gmail.com>
Cc: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/cyclades.c

@@ -1602,8 +1602,8 @@ static void cyz_handle_tx(struct cyclades_port *info,
 			info->icount.tx++;
 		}
 #endif
-ztxdone:
 		tty_wakeup(tty);
+ztxdone:
 		/* Update tx_put */
 		cy_writel(&buf_ctrl->tx_put, tx_put);
 	}