i2c: muxes: Fix return value check in mule_i2c_mux_probe()

If dev_get_regmap() fails, it returns NULL pointer not ERR_PTR(),
replace IS_ERR() with NULL pointer check, and return -ENODEV.

Fixes: d0f8e978 ("i2c: muxes: add support for tsd,mule-i2c multiplexer")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>