ppc: Remove a potential overflow in muldiv64()

The coccinelle script:
scripts/coccinelle/overflow_muldiv64.cocci
gives us a list of potential overflows in muldiv64()
(the two first parameters are 64bit values).

This patch fixes one, as the fix seems obvious:

replace muldiv64(a, b, c) by muldiv64(b, a, c)
as "a" and "b" are 64bit values but a <= NANOSECONDS_PER_SECOND.
(10^9 -> 30bit value).

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
