drivers/rtc/rtc-ds1305.c: add missing braces around sizeof

Silences the following type of warnings:

  WARNING: sizeof buf should be sizeof(buf)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.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>