wifi: rndis_wlan: fix array of flexible structures warning

Use "u8 bssid_data[]" with an appropriate cast. No binary
changes.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.0e7ce5fdbcfb.I972158734def3f93b93a3858a087cbc2cca6337e@changeid