net: mvpp2: Simplify BM pool buffers freeing

Now that all the users of mvpp2_bm_bufs_free() have been fixed, we can safely
clean the function prototype.

The function is always called to release all the buffers in a BM pool, and
the number of buffers freed is not needed. Therefore, we change the return
to a void, and remove the "num" parameter. This is a cosmetic change, to
make the code slightly cleaner.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>