powerpc/5xx: Drop unnecessary cast

do_div() does a 64-by-32 division, the 2nd parameter is a u32.

gbp->ipb_freq is already a u32, there's no need to transform it to u64
before passing it to do_div().

Signed-off-by: Wang Qing <wangqing@vivo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Add some detail to change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1644395696-3545-1-git-send-email-wangqing@vivo.com