V4L/DVB (9311): s5h1411: bugfix: Setting serial or parallel mode could destroy bits

Adding a serialmode function to read/and/or/write the register for safety.

Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>