USB: serial: add sysrq break-handler dummy

Add inline sysrq break-handler dummy to allow the compiler to eliminate
further code when either console or sysrq support isn't enabled and to
clearly mark the two sysrq functions as belonging together.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>