drivers/rtc/rtc-isl12057.c: add proper handling of oscillator failure bit

As suggested by Uwe, instead of clearing oscillator failure bit
unconditionally at driver load, this patch adds proper handling of the
flag.  The driver now returns -ENODATA when reading time from the device
and oscillator failure bit is set.  The flag is now cleared only when the
a new time value is pushed to the device.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Reported-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Peter Huewe <peter.huewe@infineon.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>