media: pxa_camera: get rid of kernel_doc warnings

Get rid of the following warnings:
    drivers/media/platform/pxa_camera.c:247: warning: No description found for parameter 'layout'
    drivers/media/platform/pxa_camera.c:867: warning: No description found for parameter 'buf'
    drivers/media/platform/pxa_camera.c:867: warning: No description found for parameter 'sg'
    drivers/media/platform/pxa_camera.c:867: warning: No description found for parameter 'sglen'
    drivers/media/platform/pxa_camera.c:867: warning: Excess function parameter 'vb' description in 'pxa_init_dma_channel'
    drivers/media/platform/pxa_camera.c:867: warning: Excess function parameter 'dma' description in 'pxa_init_dma_channel'
    drivers/media/platform/pxa_camera.c:867: warning: Excess function parameter 'cibr' description in 'pxa_init_dma_channel'
    drivers/media/platform/pxa_camera.c:1029: warning: No description found for parameter 'last_submitted'
    drivers/media/platform/pxa_camera.c:1029: warning: No description found for parameter 'last_issued'

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>