V4L/DVB (10700): saa7115: don't access reg 0x87 if it is not present.

Devices like the saa7111 do not have this register, so check for this
before using it.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>