tty: serial: amba-pl011: remove set but unused variable

tty: serial: amba-pl011: remove set but unused variable

Fix the following warning:
drivers/tty/serial/amba-pl011.c: In function check_apply_cts_event_workaround:
drivers/tty/serial/amba-pl011.c:1461:15: warning: variable dummy_read set but not used [-Wunused-but-set-variable]

The data read is useless and can be dropped.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Link: https://lore.kernel.org/r/1575619526-34482-1-git-send-email-wangxiongfeng2@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>