[media] staging: easycap: Split audio buffer and urb allocation

When the device is probed, this driver allocates
audio buffers, and audio urbs.
This patch just split this into separate functions,
which helps clearing the currently gigantic probe function.

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