serial: atmel_serial: drop low-latency workaround

Commit 1ecc26bd ("atmel_serial: split the interrupt handler") worked
around the infamous low_latency behaviour of tty_flip_buffer_push() by
dropping and reacquiring the port lock in the tasklet callback.

Since commit a9c3f68f ("tty: Fix low_latency BUG"),
tty_flip_buffer_push() always schedules a work item to push data to the
line discipline and there's no need to keep any low_latency hacks around.

Cc: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210421095509.3024-10-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>