rtc: fix build warnings in defconfigs

RTC_CLASS is changed to bool, so 'm' is invalid.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>