media: allegro: rename codec specific functions

The maximum bitrate and the size of the CPB are dependent on the level.
As the levels differ between the different codecs, the functions to get
the maximum bitrate and CPB must be codec specific, too.

Rename the functions to make it obvious that the function are only valid
for H.264.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>