USB: drop tty argument from usb_serial_handle_sysrq_char()

Since handle_sysrq() does not take tty as argument anymore we can
drop it from usb_serial_handle_sysrq_char() as well.

Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>