i40evf: tweak Tx rate params and de-magic-ify

In order for the VF to achieve its programmed Tx rate, we need to set
the max credits value to 4. While we're at it, get rid of some magic
numbers.

Change-ID: I4f17b4d3a90f1e069bdd134a543f0aa22feac3a9
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>