i40e/i40evf: fix incorrect default ITR values on driver load

The ITR register expects to be programmed in units of 2 microseconds.
Because of this, all of the drivers I40E_ITR_* constants are in terms of
this 2 microsecond register.

Unfortunately, the rx_itr_default value is expected to be programmed in
microseconds.

Effectively the driver defaults to an ITR value of half the expected
value (in terms of minimum microseconds between interrupts).

Fix this by changing the default values to be calculated using
ITR_REG_TO_USEC macro which indicates that we're converting from the
register units into microseconds.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>