[media] em28xx-audio: allocate URBs at device driver init

Instead of allocating/deallocating URBs and transfer buffers
every time stream is started/stopped, just do it once.

That reduces the memory allocation pressure and makes the
code that start/stop streaming a way simpler.

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