net: mvpp2: Fix the BM pool buffer release check

After a call to mvpp2_bm_bufs_free(), the caller usually wants to know
if the function successfully freed the requested number. However, this
cannot be done by looking into the BM pool count, because the current
buffer count was updated by mvpp2_bm_bufs_free().

In fact, the current callers of mvpp2_bm_bufs_free() use it to release
all the buffers in the pool, so we can fix this by simply checking
if the pool is not empty.

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