Avoid compiler warning about potentially unaligned pointer value

Avoid compiler warning about potentially unaligned pointer value

(&mgmt->u.deauth.reason_code + 1) is not exactly clean and now that we
have the u8 variable[] member in the struct after this field, use that
directly to avoid clang compiler warning:
ctrl_iface_ap.c:454:18: error: taking address of packed member
      'reason_code' of class or structure 'ieee80211_mgmt::(anonymous
      union)::(anonymous)' may result in an unaligned pointer value
      [-Werror,-Waddress-of-packed-member]

Signed-off-by: Jouni Malinen <j@w1.fi>