media: dvb-frontends/stv0910: Fix possible buffer overflow

Fixes smatch error:

  drivers/media/dvb-frontends/stv0910.c:715 dvbs2_nbch() error: buffer overflow 'nbch[fectype]' 2 <= 28

Also, fixes the nbch array table by adding the DUMMY_PLF element at the top
to match the enums (table element order was off by one before).

Patch sent upstream aswell.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>