media: usb: siano: Fix allocation of urbs

USB urbs must be allocated with usb_alloc_urb. Quoting the manual

Only use this function (usb_init_urb) if you _really_ understand what you
are doing.

Fix the following smatch error:

drivers/media/usb/siano/smsusb.c:53:38: warning: array of flexible structures

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>