net/enic: fix counter action

net/enic: fix counter action

- track whether counter DMAs are active or not so they can be stopped if
  needed before DMA memory is freed
- fix counter DMA shut-down by changing vnic_dev_counter_dma_cfg() to
  take the number of counters to DMA instead of high counter index and
  use num counters = 0 to shut off DMAs
- remove unnecessary checks that DMA counter memory is valid and that
  counter DMAs are in use
- change the minimum DMA period to match what 1400 series adapter is
  capable of
- fix comments and change a couple variable names to make more sense

Fixes: 86df6c4e2fce ("net/enic: support flow counter action")

Signed-off-by: John Daley <johndale@cisco.com>
Reviewed-by: Hyong Youb Kim <hyonkim@cisco.com>