ixgbevf: fix possible divide by zero in ixgbevf_update_itr

The next call to ixgbevf_update_itr will continue to dynamically
update ITR.

Copy from commit bdbeefe8 ("ixgbe: fix possible divide by zero in
ixgbe_update_itr")

Signed-off-by: Young Xiao <92siuyang@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>