i40evf: don't round Tx rate down to 0

Because the hardware configures VF Tx rates in increments of 50 Mbps,
values smaller than that would be rounded down to 0, which was
interpreted as no limit at all. Rather than do this, we round up to 50
Mbps and notify the user.

Change-ID: I5275848233fe7514cf93e11323661c68f4c38737
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>