serial: 8250_platform: Explicitly show we initialise ISA ports only once

serial8250_isa_init_ports() uses home grown approach to make itself
a singleton. Instead, explicitly show that we initialise ISA ports
once by providing a helper function which calls the original function
via DO_ONCE() macro.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240606184841.3102545-1-andriy.shevchenko@linux.intel.com


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