iwlwifi: mvm: Use div_s64 instead of do_div in iwl_mvm_ftm_rtt_smoothing()

iwlwifi: mvm: Use div_s64 instead of do_div in iwl_mvm_ftm_rtt_smoothing()

When building ARCH=arm allmodconfig:

drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c: In function ‘iwl_mvm_ftm_rtt_smoothing’:
./include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
  222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
      |                                   ^~
drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c:1070:9: note: in expansion of macro ‘do_div’
 1070 |         do_div(rtt_avg, 100);
      |         ^~~~~~

do_div() has to be used with an unsigned 64-bit integer dividend but
rtt_avg is a signed 64-bit integer.

div_s64() expects a signed 64-bit integer dividend and signed 32-bit
divisor, which fits this scenario, so use that function here to fix the
warning.

Fixes: 8b0f92549f2c ("iwlwifi: mvm: fix 32-bit build in FTM")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20211227191757.2354329-1-nathan@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>