rtc: bq4802: add error handling for devm_ioremap

When devm_ioremap fails, the lack of error-handling code may
cause unexpected results.

This patch adds error-handling code after calling devm_ioremap.

Signed-off-by: Zhouyang Jia <jiazhouyang09@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>