RDMA/core: Constify struct attribute_group

RDMA/core: Constify struct attribute_group

The only usage of the pma_table field in the ib_port struct is to pass its
address to sysfs_create_group() and sysfs_remove_group(). Make it const to
make it possible to constify a couple of static struct
attribute_group. This allows the compiler to put them in read-only memory.

Link: https://lore.kernel.org/r/20200930224004.24279-2-rikard.falkeborn@gmail.com
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>