The writeback enable logic was incorrectly implemented (due to
misunderstanding what the side effects of the implementation would be
during polling).

Fix this logic issue, while implementing a new feature allowing the user
to control the writeback frequency using the knobs for controlling
interrupt throttling that we already have.  Basically if you leave
adaptive interrupts enabled, the writeback frequency will be varied even
if busy_polling or if napi-poll is in use.  If the interrupt rates are
set to a fixed value by ethtool -C and adaptive is off, the driver will
allow the user-set interrupt rate to guide how frequently the hardware
will complete descriptors to the driver.

Effectively the user will get a control over the hardware efficiency,
allowing the choice between immediate interrupts or delayed up to a
maximum of the interrupt rate, even when interrupts are disabled
during polling.
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Co-developed-by: Brett Creeley <brett.creeley@intel.com>
Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>