i40evf: use min_t

Checkpatch complained in an earlier patch about using min(), but that
change would have been completely unrelated to the point of that patch.
So fix it here.

Change-ID: I2cd87b39cfd406850d283b88f259757a6bcd14cd
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>