Silence compiler warnings about out of array bounds indexes

Silence compiler warnings about out of array bounds indexes

struct wpa_ie_hdr had separate fields for 24-bit OUI and 8-bit oui_type
for WPA/RSN selectors. {WPA,RSN}_SELECTOR_{GET,PUT} access these four
octets through oui and the "out-of-bounds" access for the fourth octet is
actually reading/writing oui_type. This works fine, but some tools complain
about the array bounds "failure". Since oui_type is never accessed
separately, the simplest fix is to just combine these into a single 4-octet
field.