serial: pl011: Drop redundant DTR/RTS preservation on close/open

serial: pl011: Drop redundant DTR/RTS preservation on close/open

Commit d8d8ffa47783 ("amba-pl011: do not disable RTS during shutdown")
amended the PL011 serial driver to leave DTR/RTS polarity untouched on
tty close.  That change made sense.

But the commit also added code to save DTR/RTS state to an internal
variable on tty close and restore it on tty open.  That part of the
commit makes less sense:  The driver has no ->pm() callback, so the uart
remains powered after tty close and automatically preserves register
state, including DTR/RTS.

Saving and restoring registers isn't the job of the ->startup() and
->shutdown() callbacks anyway.  Rather, it should happen in ->pm().

Additionally, after pl011_startup() restores the state, the serial core
overrides it in uart_port_dtr_rts() if a baud rate has been set:

tty_port_open()
  uart_port_activate()
    uart_startup()
      uart_port_startup()
        pl011_startup()       # restores DTR/RTS from uap->old_cr
  tty_port_block_til_ready()
    tty_port_raise_dtr_rts    # if (C_BAUD(tty))
      uart_dtr_rts()
        uart_port_dtr_rts()   # raises DTR/RTS

The serial core also overrides DTR/RTS on tty close in uart_shutdown()
if C_HUPCL(tty) is set.  So a user-defined DTR/RTS polarity won't
survive a close/open cycle anyway, unless the user has set the baud rate
to zero and disabled hupcl on the tty.

Bottom line is, the code to save and restore DTR/RTS has no effect.
Remove it.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/e22089ab49e6e78822c50c8c4db46bf3ee885623.1641129328.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>