media: dvb_frontends: make static read-only array fec_tab const

The static array fec_tab is read-only so it make sense to make
it const.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>