iwlwifi: mvm: write the correct internal TXF index

The TX fifos are arranged consecutively in the SMEM, beginning
with the regular fifos, and tailed by the internal fifos.
In the current code, while trying to read the internal fifos,
we read the fifos beginning with the index zero.
By doing this we actually re-read the regular fifos.

In order to read the internal fifos, start the reading index
from the number of regular fifos configured by the fw.

Signed-off-by: Golan Ben-Ami <golan.ben.ami@intel.com>
Fixes: 39654cb3 ("iwlwifi: don't access a nonexistent register upon assert")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>