rockchip/common: Set weekday to unknown in rtc_get()

Prior to this patch, time->wday was not being initialized in rtc_get(),
but was still being used by rtc_display() to print a day.

Set to -1 which gets printed as "unknown ".

Fixes coverity issue 1357459 - Uninitialized scalar variable

Change-Id: Idecb7968f854df997b58a342e1a06a879f299394
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/15899
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/soc/rockchip/common/rk808.c

@@ -199,5 +199,7 @@ int rtc_get(struct rtc_time *time)
 	ret |= rk808_read(RTC_YEAR, &value);
 	time->year = bcd2bin(value);
 
+	time->wday = -1; /* unknown */
+
 	return ret;
 }