tty: vt, get rid of ugly FLUSH macro

It's a macro accessing and changing some local variables. And the code
uses it without appending semicolon which confuses everybody too.

Switch from this bad guy to a sane standard function.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>