wifi: rtl8xxxu: fix slab-out-of-bounds in rtl8xxxu_sta_add

In the Linux kernel, the following vulnerability has been resolved:

wifi: rtl8xxxu: fix slab-out-of-bounds in rtl8xxxu_sta_add

The driver does not set hw->sta_data_size, which causes mac80211 to allocate insufficient space for driver private station data in __sta_info_alloc(). When rtl8xxxu_sta_add() accesses members of struct rtl8xxxu_sta_info through sta->drv_priv, this results in a slab-out-of-bounds write.

KASAN report on RISC-V (VisionFive 2) with RTL8192EU adapter:

BUG: KASAN: slab-out-of-bounds in rtl8xxxu_sta_add+0x31c/0x346 Write of size 8 at addr ffffffd6d3e9ae88 by task kworker/u16:0/12

Set hw->sta_data_size to sizeof(struct rtl8xxxu_sta_info) during probe, similar to how hw->vif_data_size is configured. This ensures mac80211 allocates sufficient space for the driver's per-station private data.

Tested on StarFive VisionFive 2 v1.2A board.