RDMA/mlx5: Remove dead check for EAGAIN after alloc_mr_from_cache()

alloc_mr_from_cache() no longer returns EAGAIN, this is just dead code
now.

Fixes: aad719dc

 ("RDMA/mlx5: Allow MRs to be created in the cache synchronously")
Link: https://lore.kernel.org/r/20200914112653.345244-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>