serial_core: Move sysrq functions from header file

It's not worth to have them in every serial driver and I'm about to add
another helper function.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20200109215444.95995-2-dima@arista.com


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