serial: 8250: extract serial_get_or_create_irq_info()

serial: 8250: extract serial_get_or_create_irq_info()

This find-or-create-irq part of the serial_link_irq_chain()'s code is
logically bounded and self-standing. For easier-to-follow code flow,
extract the code to a separate function:
serial_get_or_create_irq_info().

This allows for an easier found-an-irq handling -- simple jump to the
'unlock' label and return. That results in one less 'if' levels.

Note when using guard()s in the upcoming patchset, the label can dropped
altogether.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20250611100319.186924-28-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>