wifi: iwlwifi: mvm: select ptp cross timestamp from multiple reads

wifi: iwlwifi: mvm: select ptp cross timestamp from multiple reads

iwl_mvm_get_sync_time() reads the gp2 from the device and then
reads the system clock. Since the two reads are not done atomically,
unexpected delays may happen between the two reads (e.g. context
switch) which make it inaccurate.
In order to improve the accuracy of the cross timestamp, call
iwl_mvm_get_sync_time() multiple times in a loop and take the
result in which the difference between the two clock is the smallest.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230320122330.d9e6f8f8998a.I569939ec4ddf0c6c64c112e7d0c30583f5509d9a@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>