staging: bcm2835-codec: Replace deprecated V4L2_PIX_FMT_BGR32

V4L2_PIX_FMT_BGR32 is deprecated as it is ambiguous over where
the alpha byte is.

Swap to the newer V4L2_PIX_FMT_BGRX32 format.

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