[POWERPC] Simplify rtas_change_msi() error semantics

Currently rtas_change_msi() returns either the error code from RTAS, or if
the RTAS call succeeded the number of irqs that were configured by RTAS.
This makes checking the return value more complicated than it needs to be.

Instead, have rtas_change_msi() check that the number of irqs configured by
RTAS is equal to what we requested - and return an error otherwise. This makes
the return semantics match the usual 0 for success, something else for error.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>