serial: pl011: Adopt generic flag to store auto RTS status

Back in 2010, commit 3b43816f ("ARM: 5933/1: amba-pl011: support
hardware flow control") added an autorts flag to this driver to store
whether automatic hardware flow control is requested.

In 2015, commit 391f93f2

 ("serial: core: Rework hw-assisted flow
control support") added a flag with identical functionality to generic
code.

Switch over to that flag and drop the driver-specific one.

Cc: Rabin Vincent <rabin.vincent@stericsson.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>