drivers/rtc/rtc-rk808.c: fix rtc time reading issue

After we set the GET_TIME bit, the rtc time can't be read immediately. We
should wait up to 31.25 us, about one cycle of 32khz. Otherwise reading
RTC time will return a old time. If we clear the GET_TIME bit after
setting, the time of i2c transfer is certainly more than 31.25us.

Doug said:

: I think we are safe. At 400kHz (the max speed of this part) each bit can
: be transferred no faster than 2.5us. In order to do a valid i2c
: transaction we need to _at least_ write the address of the device and the
: data onto the bus, which is 16 bits. 16 * 2.5us = 40us. That's above the
: 31.25us

[[e-mail]: tweak comment per review discussion]
Signed-off-by: Chris Zhong <[e-mail]>
Reviewed-by: Doug Anderson <[e-mail]>
Cc: Sonny Rao <[e-mail]>
Cc: Heiko Stübner <[e-mail]>
Cc: Alessandro Zummo <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/rtc/rtc-rk808.c

···
 	/* Force an update of the shadowed registers right now */
 	ret = regmap_update_bits(rk808->regmap, RK808_RTC_CTRL_REG,
 				 BIT_RTC_CTRL_REG_RTC_GET_TIME,
-				 0);
+				 BIT_RTC_CTRL_REG_RTC_GET_TIME);
 	if (ret) {
 		dev_err(dev, "Failed to update bits rtc_ctrl: %d\n", ret);
 		return ret;
 	}
 
+	/*
+	 * After we set the GET_TIME bit, the rtc time can't be read
+	 * immediately. So we should wait up to 31.25 us, about one cycle of
+	 * 32khz. If we clear the GET_TIME bit here, the time of i2c transfer
+	 * certainly more than 31.25us: 16 * 2.5us at 400kHz bus frequency.
+	 */
 	ret = regmap_update_bits(rk808->regmap, RK808_RTC_CTRL_REG,
 				 BIT_RTC_CTRL_REG_RTC_GET_TIME,
-				 BIT_RTC_CTRL_REG_RTC_GET_TIME);
+				 0);
 	if (ret) {
 		dev_err(dev, "Failed to update bits rtc_ctrl: %d\n", ret);
 		return ret;