NFSv4: Fix error handling in nfs4_sp4_select_mode()

Error code is set in the error handling cases but never used. Fix it.

Fixes: 937e3133 ("NFSv4.1: Ensure we clear the SP4_MACH_CRED flags in nfs4_sp4_select_mode()")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>