iwlwifi: mvm: remove unneeded reg write in iwl_mvm_up()

Not only that this write is not needed (as FW does this
itself), on newer HW this register is write protected
so trying to write there will cause problems.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>