[media] em28xx-audio: use bInterval on em28xx-audio

Just filling urb->interval with 1 is wrong, and causes a different
behaviour with xHCI.

With EHCI, the URB size is typically 192 bytes. However, as
xHCI specifies intervals in microframes, the URB size becomes
too short (24 bytes).

With this patch, the interval will be properly initialized, and
the device will behave the same if connected into a xHCI or an
EHCI device port.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>