rtc: snvs: fix an incorrect check of return value

rtc: snvs: fix an incorrect check of return value

[ Upstream commit 758929005f06f954b7e1c87a1c9fdb44157b228f ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>