[media] dvb-pll: Convert struct dvb_pll_desc uses to const

Convert the struct dvb_pll_desc uses to const and
change the "entries" fixed array size from 12 to []

It saves a couple KB overall and remove ~5KB of data.

$ size drivers/media/dvb-frontends/dvb-pll.o*
   text        data     bss     dec     hex filename
   8520        1552    2120   12192    2fa0 drivers/media/dvb-frontends/dvb-pll.o.new
   5624        6363    2120   14107    371b drivers/media/dvb-frontends/dvb-pll.o.old

[PG] Patch taken from https://lkml.org/lkml/2015/6/24/721

 with
commit message updated.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Michael Ira Krufky <m.krufky@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>