[media] em28xx: prefer bulk mode on webcams

Using bulk mode allows more than one webcam, as the maximum fps
is low at 640x480 resolution. So, prefer it, if the device is
a webcam.

Tested with Silvercrest 1.3 Mpixel webcam (em2710) on both bulk and isoc
modes.

Tested analog with HVR-950 model 65201/A1C0 (em2883), where only ISOC
endpoints are available for both DVB and Analog.

Tested on Hauppauge WinTV USB 2 (em2840) on both bulk and isoc modes.

It should be noticed that enabling bulk mode by default with TV boards
is a bad idea; what happens is that, while with ISOC the USB logic will
prevent the concurrent usage of two devices that spends more than 100%
of the USB2 traffic, it doesn't care with bulk transfers.

On my tests, I started two streams, one with a WinTV at 640x480x30fps
and the other one with a Silvercrest webcam at 640x480, on a lower fps)
both on bulk mode. One of the streams always silently failed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>