V4L/DVB (6235): cafe_ccic: default to allocating DMA buffers at probe time

By default, we allocate DMA buffers when actually reading from the video
capture device.  On a system with 128MB or 256MB of ram, it's very easy
for that memory to quickly become fragmented.  We've had users report
having 30+MB of memory free, but the cafe_ccic driver is still unable to
allocate DMA buffers.

Our workaround has been to make use of the 'alloc_bufs_at_load' parameter
to allocate DMA buffers during device probing.  This patch makes DMA
buffer allocation happen during device probe by default, and changes
the parameter to 'alloc_bufs_at_read'.  The camera hardware is there,
if the cafe_ccic driver is enabled/loaded it should do its best to ensure
that the camera is actually usable; delaying DMA buffer allocation
saves an insignicant amount of memory, and causes the driver to be much
less useful.

Signed-off-by: Andres Salomon <dilinger@debian.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>