octeontx2-af: Fix LBK backpressure id count

In rvu_nix_get_bpid() lbk_bpid_cnt is being read from
wrong register. Due to this backpressure enable is failing
for LBK VF32 onwards. This patch fixes that.

Fixes: fe1939bb

 ("octeontx2-af: Add SDP interface support")
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: Subbaraya Sundeep <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>