V4L/DVB: em28xx: adjust number of packets per URB

Increase the packets per URB count from 40 to 64.  I suspect that whoever was
looking at the usbsnoop captures saw "0x40" packets and mistook it for "40".

As a result of this change, I can see a 25% reduction in the number of
interrupts generated via powertop.

This work was sponsored by EyeMagnet Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>