ixgbe: Wait for master disable to be set

According to the datasheets, the driver should wait for the master
disable bit to read as being set before checking the status
register for master disable.

Reported-by: Dan Streetman <dan.streetman@canonical.com>
Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Darin Miller <darin.j.miller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>