Some chipsets have several problems when pci to pci transfers are activated
on overlay mode. the option no_overlay allows disabling such feature of
the driver, in favor of keeping the system stable.
The default is to use pcipci_fail flag defined on drivers/pci/quirks.c.
It also allows the user to override it by forcing disable overlay or forcing
enable. Forcing enable may generate PCI transfer corruption, including disk
mass corruption, so should be used with care.
Added a text description to this option and make messages looks the same at
both bttv and saa7134 drivers.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>