ixgbe: turn off MDD while modifying SRRCTL

Modifying SRRCTL register can generate MDD event.

Turn MDD off during SRRCTL register write to prevent generating MDD.

Fix RCT in ixgbe_set_rx_drop_en().

Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Radoslaw Tyl <radoslawx.tyl@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>