Some applications (xawtv, qv4l2) mix read and mmap calls. Allow switching
from read mode back to mmap mode (by doing a reqbufs).
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Acked-by: NJean-Francois Moine <moinejf@free.fr>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>