RDMA/rtrs: client: Fix function return on success

Remove the if-statement and return the value contained in _err_,
unconditionally.

Link: https://lore.kernel.org/r/20200519163612.GA6043@embeddedor


Addresses-Coverity-ID: 1493753 ("Identical code for different branches")
Fixes: 6a98d71d ("RDMA/rtrs: client: main functionality")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>