nfp: calculate ring masks without conditionals

Ring enable masks are 64bit long.  Replace mask calculation from:
  block_cnt == 64 ? 0xffffffffffffffffULL : (1 << block_cnt) - 1
with:
  (U64_MAX >> (64 - block_cnt))
to simplify the code.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>