RDMA/mlx5: Remove redundant assignment to ret

RDMA/mlx5: Remove redundant assignment to ret

Variable 'ret' is set to the rerurn value of function
mlx5_mr_cache_alloc() but this value is never read as it is overwritten
with a new value later on, hence it is a redundant assignment and can be
removed

Clean up the following clang-analyzer warning:

 drivers/infiniband/hw/mlx5/odp.c:421:2: warning: Value stored to 'ret' is never read [clang-analyzer-deadcode.DeadStores]

Fixes: 419ecd34d0b8 ("RDMA/mlx5: Consolidate MR destruction to mlx5_ib_dereg_mr()")
Link: https://lore.kernel.org/r/1620296001-120406-1-git-send-email-yang.lee@linux.alibaba.com
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>