i40e: tighten up ring enable/disable flow

Change the do/while to a for loop, so we don't hit the delay each
time, even when the register is ready for action.
Don't bother to set or clear the QENA_STAT bit as it is
read-only.

Change-ID: Ie464718804dd79f6d726f291caa9b0c872b49978
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>