nl80211: Fix potential NULL pointer dereference in set_ap()

nl80211: Fix potential NULL pointer dereference in set_ap()

In the code review, it was found that param->freq is accessed without
NULL check in wpa_driver_nl80211_set_ap(), while in other sections of
the code, freq is accessed only after NULL validation. This situation
could result in a segmentation fault at least in theory.

Add a NULL check for freq before accessing it to be consistent with the
other uses.

Fixes: 0c6c948047de ("nl80211: Support setting up an AP on a specified link")
Signed-off-by: Karthikeyan Kathirvel <quic_kathirve@quicinc.com>