serial: add parameter to force skipping the test for the TXEN bug

Allow users to force skipping the TXEN test at init time. Applies
to all serial ports. Intended for debugging only.

There is a blacklist for devices where we need to skip the test but the
list is not complete.  This lets users force skipping the test so we can
determine if they need to be added to the list.

Some HP machines with weird serial consoles have this problem and there
may be more.

Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>