serial: st-asc: fix typo in property name

Changes the property name read in the driver according to the YAML.
According to device-tree documentation, property names should not
include underscores.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Message-ID: <20230604083558.16661-1-rgallaispou@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>