powerpc/powernv: Add a null pointer check to scom_debug_init_one()

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.
Add a null pointer check, and release 'ent' to avoid memory leaks.

Fixes: bfd2f0d4 ("powerpc/powernv: Get rid of old scom_controller abstraction")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231208085937.107210-1-chentao@kylinos.cn