RDMA/nes: Fix compiler warning at nes_verbs.c:1955

Initialize pbl_count_256 to 0 to get rid of the warning:

    drivers/infiniband/hw/nes/nes_verbs.c: In function 'nes_reg_mr':
    drivers/infiniband/hw/nes/nes_verbs.c:1955: warning: 'pbl_count_256' may be used uninitialized in this function

Reported-by: Roland Dreier <rdreier@cisco.com>
Signed-off-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>