serial: 8250: introduce RPM guard()s

serial: 8250: introduce RPM guard()s

Having this, guards like these work:
  guard(serial8250_rpm)(up);
or
  scoped_guard(serial8250_rpm, up) {
    ...
  }

See e.g. "serial: 8250: use guard()s" later in this series.

And make them available to anyone (EXPORT + put in 8250.h) as drivers
open code this anyway.

The _tx ones are not defined as they would have no user.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250814072456.182853-5-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>