lib/rtc: Fix off-by-one error in February day count in leap year

lib/rtc: Fix off-by-one error in February day count in leap year

The month argument passed to rtc_month_days is 0-based, not 1-based.
This results in the RTC being reverted to the build date constantly
on 29th February 2024.

Change-Id: If451e3e3471fef0d429e255cf297050a525ca1a2
Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80790
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Michał Kopeć <michal.kopec@3mdeb.com>
(cherry picked from commit adf042f6c61e92c181171118768e4309d87146b1)
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80823