tty: st-asc: Update tty alias

Since dtc v1.4.6-9-gaadd0b65c987, aliases property name
must include only lowercase and '-'.

After having updated all STi boards serial aliases from "ttyASN"
to "serialN", st-asc driver need to be updated accordingly as tty
aliases id is retrieved using of_alias_get_id().

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>