media: venus: venc: set correctly GOP size and number of B-frames

This change fixes the calculation of B-frames and GOP size by
adopt v4l2 controls with the firmware interface expectations.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: fixed two small checkpatch comments]
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>