RDMA/srpt: Fix typo in srpt_unregister_mad_agent docstring

htmldocs fails with:

drivers/infiniband/ulp/srpt/ib_srpt.c:630: warning: Function parameter or member 'port_cnt' not described in 'srpt_unregister_mad_agent'

Fixes: 372a1786 ("IB/srpt: Fix memory leak in srpt_add_one")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>