iwlwifi: mvm: fix 32-bit build in FTM

On a 32-bit build, the division here needs to be done
using do_div(), otherwise the compiler will try to call
a function that doesn't exist, thus failing to build.

Fixes: b68bd2e3 ("iwlwifi: mvm: Add FTM initiator RTT smoothing logic")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211219111352.e56cbf614a4d.Ib98004ccd2c7a55fd883a8ea7eebd810f406dec6@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>