powerpc/fadump: Use swap() instead of open coding it

Clean the following coccicheck warning:

./arch/powerpc/kernel/fadump.c:1291:34-35: WARNING opportunity for
swap().

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220129034847.76902-1-jiapeng.chong@linux.alibaba.com