RDMA/siw: Use max() instead of doing it manually

Fix following coccicheck warning:

./drivers/infiniband/sw/siw/siw_verbs.c:665:28-29: WARNING opportunity for max().

Link: https://lore.kernel.org/r/1638439679-114250-1-git-send-email-jiapeng.chong@linux.alibaba.com


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>