IB/srp: Print "ib_srp: " prefix once

pr_debug() already prints prefix PFX. Avoid that PFX is printed
twice if the debug statement in srp_add_target() is enabled.

Fixes: 34aa654e ("IB/srp: Avoid that I/O hangs due to a cable pull during LUN scanning")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>