rtc-ds1390: fix compilation warnings in drivers/rtc/rtc-ds1390.c

drivers/rtc/rtc-ds1390.c:125: warning: unused variable 'rtc'

Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>