iwlwifi: pcie: don't reset TXQ write pointer

Currently code sets the write pointer when getting the TX queue
allocate response. This causes a redundant interrupt with any actual
change in the pointer. Remove this write altogether.

Fixes: 310181ec ("iwlwifi: move to TVQM mode")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>