media: bcm2835-unicam: Fix one-to-many mapping for YUYV formats

V4L2 format V4L2_PIX_FMT_YUYV maps to both MEDIA_BUS_FMT_YUYV8_2X8
and MEDIA_BUS_FMT_YUYV8_1X16. The change to not cache the active
formats also meant that we only ever found the first variant of
the mediabus format when trying to setup the device.

Flag the formats that have other representations, and ensure
that the format conversion checks whether the found format
matches one supported by the sensor before returning it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>