iwlwifi: expose fw usniffer mode to more utilities

Today, in order to configure fw in usniffer mode, the ucode
must have the corresponding tlv, which is revealed to the driver
while parsing the ucode.

Expose the mode of the usniffer to other utilities in the driver
(other than the ucode parser) by passing back a pointer to the value.
This can be very useful for allowing configuring the fw dbg data
using an external configuration file, because this configuration
depends on the fw usniffer mode.

Signed-off-by: Golan Ben-Ami <golan.ben.ami@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>