i40e: Fix Adaptive ITR enabling

This patch fixes a bug introduced with the addition of the per queue
ITR feature support in ethtool.  With that addition, there were
functions added which converted the ITR settings to binary values.
The IS_ENABLED macros that run on those values check whether a bit
is set or not and with the value being binary, the bit check always
returned ITR disabled which prevents any updating of the ITR rate.
This patch fixes the problem by changing the functions to return the
current ITR value instead and renaming it to better reflect
its function.  These functions now provide a value which will be
accurately asessed and update the ITR as intended.

Change-ID: I14f1d088d052e27f652aaa3113e186415ddea1fc
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>