devm_kstrdup() returns pointer to allocated string on success,
NULL on failure. So it is better to check the return value of it.

Fixes: e35478ea ("i2c: mux: demux-pinctrl: run properly with multiple instances")
Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>