RDMA/iwpm: Remove redundant initialization of pointer err_str

The pointer err_str is being initialized with a value that is never read,
it is being updated later on. The assignment is redundant and can be
removed.

Link: https://lore.kernel.org/r/20211007173942.21933-1-colin.king@canonical.com


Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>