rtc: rv3028: Add missed check for devm_regmap_init_i2c()

rv3028_probe() misses a check for devm_regmap_init_i2c().
Add the missed check to fix it.

Fixes: e6e7376c ("rtc: rv3028: add new driver")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200528103950.912353-1-hslester96@gmail.com