iwlwifi: pcie: write to legacy register also in MQ

Due to hardware bug, upon any shadow free-queue register write
access, a legacy RBD shadow register must be written as well.
This is required in order to trigger a copy of the shadow registers
values after MAC exits sleep state.
Specifically, the driver has to write (any value) to the legacy RBD
register each time FRBDCB is accessed.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>