i2c: mux: demux-pinctrl: check the return value of devm_kstrdup()

[ Upstream commit 7c0195fa ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>