ACPI / LPSS: fix UART Auto Flow Control

There is an additional bit in the GENERAL register on newer
silicon that needs to be set or UART's RTS pin fails to
reflect the flow control settings in the Modem Control
Register.

This will fix an issue where the RTS pin of the UART stays
always at 1.8V, regardless of the register settings.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>