[media] em28xx: Change scope of em28xx-input local functions to static

This functions are no longer used from another file,
so they should be declared as static.
Also is it necessary to move some of them before they
are used, since they are no longer header-declared.

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