serial: stm32: update conflicting RTS/CTS config comment

serial: stm32: update conflicting RTS/CTS config comment

The comment for conflicting RTS/CTS config refers to "st, hw-flow-ctrl",
but this property is deprecated since the generic RTS/CTS property has
been introduced by the patch 'serial: stm32: Use generic DT binding for
announcing RTS/CTS lines'.
Update the comment to refer to both generic and deprecated RTS/CTS
properties.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210106162203.28854-7-erwan.leray@foss.st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>