V4L/DVB (9447): stb6100: improve rounding

Optimization: Round the requested value to achieve a
+/-1MHz error instead of +0/-2MHz

Signed-off-by: Reinhard Nissl <rnissl@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>