IB/srp: Avoid using uninitialized variable

We might return res which is not initialized. Also
reduce code duplication by exporting srp_parse_tmo so
srp_tmo_set can reuse it.

Detected by Coverity.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Jenny Falkovich <jennyf@mellanox.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>