P2P: Iterate through full pref_chan list in search of a valid channel

P2P: Iterate through full pref_chan list in search of a valid channel

p2p_get_pref_freq() went through the full list only if the channels
arguments was provided. If no channel list contraint was in place, the
first pref_chan item was picked regardless of whether it is valid
channel and as such, a later valid entry could have been ignored. Allow
this to loop through all the entries until a valid channel is found or
the end of the pref_chan list is reached. As an extra bonus, this
simplifies the p2p_get_pref_freq() implementation quite a bit.

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