powerpc/xive: Replace msleep(x) with msleep(OPAL_BUSY_DELAY_MS)

Replace msleep(x) with with msleep(OPAL_BUSY_DELAY_MS) to document
these sleeps are to wait for opal (firmware).

Signed-off-by: Daniel Klamt <eleon@ele0n.de>
Signed-off-by: Bjoern Noetel <bjoern@br3ak3r.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>