[media] em28xx: make em28xx_set_outfmt() working with EM25xx family bridges

Streaming doesn't work with the EM2765 if bit 5 of the output format register
0x27 is set.
It's actually not clear if really has to be set for the other chips, but for
now let's keep it to avoid regressions and add a comment to the code.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>