drivers: staging: bcm2835-isp: Respect caller's stride value

The stride value reported for output image buffers should be at least
as large as any value that was passed in by the caller (subject to
correct alignment for the pixel format). If the value is zero (meaning
no value was passed), or is too small, the minimum acceptable value
will be substituted.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>