rtc: snvs: fix an incorrect check of return value

Function devm_regmap_init_mmio() returns an ERR_PTR on error. However,
in function snvs_rtc_probe() its return value is checked against NULL.
This patch fixes it by checking the return value with IS_ERR().

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>