mac80211: use ieee802_11_parse_elems() in ieee80211_prep_channel()

In function ieee80211_prep_channel(), it has some ieee80211_bss_get_ie()
and cfg80211_find_ext_ie() to get the IE, this is to use another
function ieee802_11_parse_elems() to get all the IEs in one time.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Link: https://lore.kernel.org/r/20210924100052.32029-6-wgong@codeaurora.org


[remove now unnecessary size validation, use -ENOMEM, free elems earlier
 for less error handling code]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>