powerpc: Avoid ignoring copyin()'s return value

A recent change made it possible for cpu_set_upcall() to return an
error. Do that here instead of ignoring an error from copyin().

Reviewed by: jhibbits
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D43105

(cherry picked from commit bdf03b4bcc4a9aa0be503dbc64415e6b0b845fbc)