iwlwifi: pcie: don't init a Tx queue with an SSN > size of the queue

The TVQM tells us the initial write pointer for a queue,
but that write pointer is in WiFi sequence number unit
and not in TFD index unit. Which means that the write
pointer in the TVQM's response can be bigger than the
Tx queue ring size.

Fix that by modulo'ing the write pointer from the TVQM
with the Tx queue size.

Fixes: 66128fa08806 ("iwlwifi: move to TVQM mode")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>