powerpc/opal: Annotate out param endianness

Sparse reports an endian mismatch with args to opal_int_get_xirr().
Checking the skiboot source[1] shows the function takes a __be32*
(as expected), so update the function declaration to reflect this.

[1]: https://github.com/open-power/skiboot/blob/80e2b1dc73/hw/xive.c#L3479



Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-9-bgray@linux.ibm.com