mfd: syscon: Fix null pointer dereference in of_syscon_register()

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

Fixes: e15d7f2b

 ("mfd: syscon: Use a unique name with regmap_config")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231204092443.2462115-1-chentao@kylinos.cn
Signed-off-by: Lee Jones <lee@kernel.org>