i40e: initialize ITRN registers with correct values

i40e: initialize ITRN registers with correct values

Since commit 7e3162863b40 ("i40e/i40evf: Use usec value instead of reg
value for ITR defines") the driver tracks the interrupt throttling
intervals in single usec units, although the actual ITRN/ITR0 registers are
programmed in 2 usec units. Most register programming flows in the driver
correctly handle the conversion, although it is currently not applied when
the registers are initialized to their default values. Most of the time
this doesn't present a problem since the default values are usually
immediately overwritten through the standard adaptive throttling mechanism,
or updated manually by the user, but if adaptive throttling is disabled and
the interval values are left alone then the incorrect value will persist.

Since the intended default interval of 50 usecs (vs. 100 usecs as
programmed) performs better for most traffic workloads, this can lead to
performance regressions.

This patch adds the correct conversion when writing the initial values to
the ITRN registers.

Signed-off-by: Nicholas Nunley <nicholas.d.nunley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>