staging: rtl8723au: Use _FIXED_IE_LEN_ instead of hard coded value

This should help catching all the users once we stop carrying the
fixed beacon elements in the IE array.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>