cfg80211: add helper to find an IE that matches a byte-array

There are a few places where an IE that matches not only the EID, but
also other bytes inside the element, needs to be found.  To simplify
that and reduce the amount of similar code, implement a new helper
function to match the EID and an extra array of bytes.

Additionally, simplify cfg80211_find_vendor_ie() by using the new
match function.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>