wifi: mac80211: fix beacon interval calculation overflow

wifi: mac80211: fix beacon interval calculation overflow

[ Upstream commit 7a3750ff0f2e8fee338a9c168f429f6c37f0e820 ]

As we are converting from TU to usecs, a beacon interval of
100*1024 usecs will lead to integer wrapping. To fix change
to use a u32.

Fixes: 057d5f4ba1e4 ("mac80211: sync dtim_count to TSF")
Signed-off-by: Lachlan Hodges <lachlan.hodges@morsemicro.com>
Link: https://patch.msgid.link/20250621123209.511796-1-lachlan.hodges@morsemicro.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>