IB/hfi1: Missing return value in error path for user sdma

If the set_txreq_header_agh() function returns an error, the exit path
is chosen.

In this path, the code fails to set the return value.  This will cause
the caller to not realize an error has occurred.

Set the return value correctly in the error path.

Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>