[media] em28xx: use the proper prefix for board names

All boards use EM28xxx_BOARD, to identify that the macro
refers to a card entry. So:

EM2874_LEADERSHIP_ISDBT -> EM2874_BOARD_LEADERSHIP_ISDBT

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