ixgbe: fix possible divide by zero in ixgbe_update_itr

Protect the code by bailing out of ixgbe_update_itr() when this occurs.
The next call to ixgbe_update_itr will continue to dynamically update ITR.

Signed-of-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>