drivers/rtc/rtc-ds1286.c: fix compiler warning while doing make W=1

below is the warning reported with the gcc (gcc (GCC) 4.7.2 20121109
(Red Hat 4.7.2-8)) with make W=1:

  drivers/rtc/rtc-ds1286.c: In function `ds1286_read_alarm':
  drivers/rtc/rtc-ds1286.c:273:16: warning: variable `cmd' set but not used [-Wunused-but-set-variable]

fixed by removing the variable

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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>