RDMA/uverbs: Remove redundant assignments

The kbuild reported the following warning, so clean whole uverbs_cmd.c
file.

   drivers/infiniband/core/uverbs_cmd.c:1066:6: warning: Variable 'ret' is reassigned a value before the old one has been used. [redundantAssignment]
    ret = uverbs_request(attrs, &cmd, sizeof(cmd));
        ^
   drivers/infiniband/core/uverbs_cmd.c:1064:0: note: Variable 'ret' is reassigned a value before the old one has been used.
    int    ret = -EINVAL;
   ^

Link: https://lore.kernel.org/r/20200720175627.1273096-2-leon@kernel.org


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>