[media] mem2mem_testdev: use 40ms default transfer time

The default of 1 second is a bit painful, switch to a 25 Hz framerate.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>