net: systemport: Fix coalescing settings handling

There were a number of issues with setting the RX coalescing parameters:

- we would not be preserving values that would have been configured
  across close/open calls, instead we would always reset to no timeout
  and 1 interrupt per packet, this would also prevent DIM from setting its
  default usec/pkts values

- when adaptive RX would be turned on, we woud not be fetching the
  default parameters, we would stay with no timeout/1 packet per
  interrupt until the estimator kicks in and changes that

- finally disabling adaptive RX coalescing while providing parameters
  would not be honored, and we would stay with whatever DIM had
  previously determined instead of the user requested parameters

Fixes: b6e0e875 ("net: systemport: Implement adaptive interrupt coalescing")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tal Gilboa <talgi@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>