iwlwifi: fix 64-bit division

do_div() expects unsigned operands and otherwise triggers a warning like:

drivers/net/wireless/intel/iwlwifi/mvm/ftm-initiator.c:465:2: error: comparison of distinct pointer types ('typeof ((rtt_avg)) *' (aka 'long long *') and 'uint64_t *' (aka 'unsigned long long *')) [-Werror,-Wcompare-distinct-pointer-types]
        do_div(rtt_avg, 6666);
        ^~~~~~~~~~~~~~~~~~~~~
include/asm-generic/div64.h:222:28: note: expanded from macro 'do_div'
        (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
               ~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~
1 error generated.

Change the do_div() to the simpler div_s64() that can handle
negative inputs correctly.

Fixes: 937b10c0 ("iwlwifi: mvm: add debug prints for FTM")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>