rtc: Check return value from mc146818_get_time()

There are 4 users of mc146818_get_time() and none of them was checking
the return value from this function. Change this.

Print the appropriate warnings in callers of mc146818_get_time() instead
of in the function mc146818_get_time() itself, in order not to add
strings to rtc-mc146818-lib.c, which is kind of a library.

The callers of alpha_rtc_read_time() and cmos_read_time() may use the
contents of (struct rtc_time *) even when the functions return a failure
code. Therefore, set the contents of (struct rtc_time *) to 0x00,
which looks more sensible then 0xff and aligns with the (possibly
stale?) comment in cmos_read_time:

	/*
	 * If pm_trace abused the RTC for storage, set the timespec to 0,
	 * which tells the caller that this RTC value is unusable.
	 */

For consistency, do this in mc146818_get_time().

Note: hpet_rtc_interrupt() may call mc146818_get_time() many times a
second. It is very unlikely, though, that the RTC suddenly stops
working and mc146818_get_time() would consistently fail.

Only compile-tested on alpha.

Signed-off-by: Mateusz Jończyk <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Alexandre Belloni <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Alexandre Belloni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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