mfd: sta2x11-mfd: Fix return value check in sta2x11_mfd_platform_probe()

In case of error, the function devm_regmap_init_mmio() returns
ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Lee Jones <lee.jones@linaro.org>