[media] em28xx: Make card_setup() and pre_card_setup() static

This cleans namespace a bit by making em28xx_card_setup()
em28xx_pre_card_setup() static functions.

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