ixgbe: clean up the condition for turning on/off the laser

This patch simplifies the check for calling en/disable_tx_laser() function
pointer. The pointer is only set on parts that can use it.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>