powerpc/powernv: Add a null pointer check in opal_powercap_init()

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

Fixes: b9ef7b4b ("powerpc: Convert to using %pOFn instead of device_node.name")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231126095739.1501990-1-chentao@kylinos.cn