RDMA/nldev: Split nldev_stat_set_mode_doit out of nldev_stat_set_doit

In order to allow expansion of the set command with more set options, take
the set mode out of the main set function.

Link: https://lore.kernel.org/r/20211008122439.166063-9-markzhang@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>