V4L/DVB (3963): Em28xx/: possible cleanups

This patch contains the following possible cleanups:
- make the following needlessly global functions static:
  - em28xx-core.c: em28xx_accumulator_set()
  - em28xx-core.c: em28xx_capture_area_set()
  - em28xx-core.c: em28xx_scaler_set()
  - em28xx-core.c: em28xx_isocIrq()
- remove the following unused EXPORT_SYMBOL's:
  - em28xx-cards.c: em28xx_boards
  - em28xx-cards.c: em28xx_bcount
  - em28xx-cards.c: em28xx_id_table

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>