mfd: tps6594: Add null pointer check to tps6594_device_init()

devm_kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Fixes: 325bec71

 ("mfd: tps6594: Add driver for TI TPS6594 PMIC")
Suggested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20231208033320.49345-1-chentao@kylinos.cn
Signed-off-by: Lee Jones <lee@kernel.org>