net: dsa: vsc73xx: Add null pointer check to vsc73xx_gpio_probe

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

Fixes: 05bd97fc

 ("net: dsa: Add Vitesse VSC73xx DSA router driver")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240111072018.75971-1-chentao@kylinos.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>