media: dib8000: rewrite the init prbs logic

The logic at dib8000_get_init_prbs() has a few issues:

1. the tables used there has an extra unused value at the beginning;
2. the dprintk() message doesn't write the right value when
   transmission mode is not 8K;
3. the array overflow validation is done by the callers.

Rewrite the code to fix such issues.

This should also shut up those smatch warnings:

	drivers/media/dvb-frontends/dib8000.c:2125 dib8000_get_init_prbs() error: buffer overflow 'lut_prbs_8k' 14 <= 14
	drivers/media/dvb-frontends/dib8000.c:2129 dib8000_get_init_prbs() error: buffer overflow 'lut_prbs_2k' 14 <= 14
	drivers/media/dvb-frontends/dib8000.c:2131 dib8000_get_init_prbs() error: buffer overflow 'lut_prbs_4k' 14 <= 14
	drivers/media/dvb-frontends/dib8000.c:2134 dib8000_get_init_prbs() error: buffer overflow 'lut_prbs_8k' 14 <= 14

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>