rtc: sc27xx: Remove unnecessary conversion to bool

Here we could use the '!=' expression to fix the following coccicheck
warning:

./drivers/rtc/rtc-sc27xx.c:566:50-55: WARNING: conversion to bool not needed here
Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1604647854-876-1-git-send-email-kaixuxia@tencent.com

drivers/rtc/rtc-sc27xx.c

@@ -563,7 +563,7 @@ static int sprd_rtc_check_power_down(struct sprd_rtc *rtc)
 	 * means the RTC has been powered down, so the RTC time values are
 	 * invalid.
 	 */
-	rtc->valid = val == SPRD_RTC_POWER_RESET_VALUE ? false : true;
+	rtc->valid = val != SPRD_RTC_POWER_RESET_VALUE;
 	return 0;
 }