ixgbe: Fix minor typo while freeing irq

The array subscript increments after the execution of the statement.
So there is no issue here. However it helps to read the code better.

Signed-off-by: Babu Moger <babu.moger@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>