drivers/rtc/rtc-isl12057.c: report error code upon failure in dev_err() calls

As pointed out by Mark, it is generally useful to log the error code when
reporting a failure.  This patch improves existing calls to dev_err() in
ISL12057 driver to also report error code.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Suggested-by: Mark Brown <broonie@kernel.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: Grant Likely <grant.likely@linaro.org>
Acked-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>