i40e/i40evf: rename bytes_per_int to bytes_per_usec

i40e/i40evf: rename bytes_per_int to bytes_per_usec

This value is not calculating bytes_per_int, which would actually just
be bytes/ITR_COUNTDOWN_START, but rather it's calculating bytes/usecs.

Rename the variable for clarity so that future developers understand
what the value is actually calculating.

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>