iwlwifi: pcie: gen2: fix race in cmd fifo write ptr

Avoid a race where two (or more) commands get the
same index:

1. T1 calls enqueue_hcmd and the local TFD index is assigned to
   txq->write_ptr;
2. Context switch 'before incrementing txq->write_ptr';
3. T2 calls enqueue_hcmd and the local TFD index is assigned to
   txq->write_ptr;
4. Now the index is set to the same value for both commands of T1 and
   T2.

To prevent this from happening, set the local TFD index inside the
critical section (the index is set by global txq write pointer).

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>