wlantest: Try harder to find a STA entry with PTK for 4-address frames

wlantest: Try harder to find a STA entry with PTK for 4-address frames

Commit aab66128369c5953e70f867e997a54146bcca88b ('wlantest: Search
bss/sta entry more thoroughly for 4-address frames') allowed wlantest to
find a STA entry in this type of cases, but it was still possible for
that STA entry to be the one that has no derived PTK while the STA entry
for the other side of the link might have the derived PTK available.

Extend this BSS/STA selection mechanism to use sta->ptk_set to determine
which STA entry is more useful for decryption, i.e., select the one with
a known PTK.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>