drivers/rtc/rtc-isl1208.c: use devm_* APIs

devm_* APIs are device managed and make code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Herbert Valerio Riedel <hvr@gnu.org>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>