firmware/dmi_scan: Uninline dmi_get_bios_year() helper

Uninline dmi_get_bios_year() which, in particular, allows us
to optimize it in the future.

While doing this, convert the function to return an error code
when BIOS date is not present or not parsable, or CONFIG_DMI=n.

Additionally, during the move, add a bit of documentation.

Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rafael J . Wysocki <rjw@rjwysocki.net>
Cc: linux-acpi@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Fixes: 492a1abd

 ("dmi: Introduce the dmi_get_bios_year() helper function")
Signed-off-by: Ingo Molnar <mingo@kernel.org>