ixgbe: Check whether FDIRCMD writes actually complete

Wait up to about 100 us for FDIRCMD writes to complete and return
failure indications.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>