[media] staging: easycap: Split device struct alloc and retrieval code

When the device is probed a driver struct is either
allocated or retrieved.
This operation is logically splitted in several functions.

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