wifi: cw1200: Avoid processing an invalid TIM IE

While converting struct ieee80211_tim_ie::virtual_map to be a flexible
array it was observed that the TIM IE processing in cw1200_rx_cb()
could potentially process a malformed IE in a manner that could result
in a buffer over-read. Add logic to verify that the TIM IE length is
large enough to hold a valid TIM payload before processing it.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230831-ieee80211_tim_ie-v3-1-e10ff584ab5d@quicinc.com