iwlwifi: pcie: allow the op_mode to block the tx queues

In certain flows (see next patches), the op_mode may need to
block the Tx queues for a short period. Provide an API for
that. The transport is in charge of counting the number of
times the queues are blocked since the op_mode may block the
queues several times in a row before unblocking them.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>