samples: video: tcpserversink: fix stm32 venc max buffer size

The CONFIG_VIDEO_BUFFER_POOL_SZ_MAX represent the size of the
biggest buffer in the pool size and not the whole pool size.
For the stm32-venc this should be 1-000-000 and not 10-000-000.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>