i2c: mux: pinctrl: potential NULL dereference on error

If i2c_mux_alloc() fails then we'd have a NULL dereference here.

Fixes: c4aee3e1 ("i2c: mux: pinctrl: remove platform_data")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peter Rosin <peda@axentia.se>