cxgbi: Typo in MODULE_PARM_DESC

The module_param is "cxgb3i_rx_credit_thres" so the MODULE_PARM_DESC()
should match that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>