[media] em28xx: add module parameter for selection of the preferred USB transfer type

By default, isoc transfers are used if possible.
With the new module parameter, bulk can be selected as the
preferred USB transfer type.

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