[media] media: em28xx: Remove unused urb arrays from device struct

These arrays were embedded in the struct itself, but they weren't
used by anyone, since urbs are now dinamically allocated
at em28xx_usb_isoc_ctl struct.
Tested by compilation only.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>