pNFS: Fix assignment of xprtdata.cred

The comma at the end of the line was leftover from an earlier refactor
of the _nfs4_pnfs_v3_ds_connect() function. This is technically valid C,
so the compilers didn't catch it, but if I'm understanding how it works
correctly it assigns the return value of rpc_clnt_add_xprtr() to
xprtdata.cred.

Reported-by: Olga Kornievskaia <kolga@netapp.com>
Fixes: a12f996d ("NFSv4/pNFS: Use connections to a DS that are all of the same protocol family")
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>