drivers/rtc/interface.c: __rtc_read_time: reduce log level

drivers/rtc/interface.c: __rtc_read_time: reduce log level

__rtc_read_time logs should be debug logs instead of error logs.

For example, when the RTC clock is not set, it's not really useful
to print a kernel error log every time someone tries to read the clock:

	~ # hwclock -r
	[  604.508263] rtc rtc0: read_time: fail to read
	hwclock: RTC_RD_TIME: Invalid argument

If there's a real error, it's likely that lower level or higher level
code will tell it anyway. Make these logs debug logs, and also print
the error code for the read failure.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>