tty: atmel_serial: update outdated reference to atmel_tasklet_func()

tty: atmel_serial: update outdated reference to atmel_tasklet_func()

The modem-status comparison that used irq_status_prev was
moved from atmel_tasklet_func() into atmel_handle_status() in
commit d033e82db9a5 ("tty/serial: at91: handle IRQ status
more safely").  Update the comment accordingly.

Assisted-by: unnamed:deepseek-v3.2 coccinelle
Signed-off-by: Kexin Sun <kexinsun@smail.nju.edu.cn>
Link: https://patch.msgid.link/20260324024857.3244-1-kexinsun@smail.nju.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>