iwlwifi: don't access a nonexistent register upon assert

The commit below added code to dump the content of FIFOs
that are present only on dual CPU products (8000 and up).
This broke 7265D whose firmware does advertise
IWL_UCODE_TLV_CAPA_EXTEND_SHARED_MEM_CFG but doesn't have 2
CPUs. The current code does check the length of the FIFO
before dumping them (and the nonexistent FIFO has a 0
length), but we still accessed a register to set the FIFO
number and that made the DMA unhappy.

The impact was a much longer recovery upon firmware assert.

Fixes: 5b086414 ("iwlwifi: mvm: support dumping UMAC internal txfifos")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>