ixgbe: handle parameters for tx and rx EITR, no div0

The driver was doing a divide by zero when adjusting tx-usecs.
This patch removes the divide by zero code and changes the logic slightly
to ignore tx-usecs in the case of shared TxRx vectors.

Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>