serial: 8250_dw: remove clock-notifier infrastructure

serial: 8250_dw: remove clock-notifier infrastructure

The clock notifier and matching work_struct in dw8250_data were added
in 2020 for the Baikal-T1 SoC, whose multiple UART ports share a
single reference clock and need to be informed when another consumer
re-rates that clock.

Baikal SoC support has since been removed from the kernel (see e.g.
commit 5d6c477687ae ("clk: baikal-t1: Remove not-going-to-be-supported
code for Baikal SoC") and the matching removals across bus/, mtd/,
PCI/, hwmon/, memory/). No remaining in-tree user needs the
cross-device baudclk rate-change notification path: the only
configuration that wired up the notifier was Baikal-T1's shared
reference clock topology.

Drop the now-unused clock-notifier and its deferred-update worker:

  - struct dw8250_data fields clk_notifier and clk_work,
  - the clk_to_dw8250_data() and work_to_dw8250_data() helpers,
  - the dw8250_clk_work_cb() and dw8250_clk_notifier_cb() callbacks,
  - the INIT_WORK / notifier_call setup in dw8250_probe(),
  - the clk_notifier_register() / queue_work() in dw8250_probe(),
  - the matching clk_notifier_unregister() / flush_work() in
    dw8250_remove(),
  - the stale comment in dw8250_set_termios() about the worker
    blocking,
  - the linux/notifier.h and linux/workqueue.h includes that are
    no longer used.

dw8250_set_termios() keeps calling clk_set_rate() directly, which is
all the remaining single-UART configurations require.

Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260514143746.23671-3-sozdayvek@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>