staging/rdma/hfi1: Fix module parameter spelling

Fix the spelling of user_credit_return_threshold, it was incorrectly
spelled as user_credit_return_theshold causing two module parameters,
one with typo, to be shown in modinfo

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>