[media] DocBook: in non-blocking mode return EAGAIN in hwseek

VIDIOC_S_HW_FREQ_SEEK should return EAGAIN when called in non-blocking
mode. This might change in the future if we add support for this in the
future, but right now this is not supported.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>