[SCSI] mptfusion: unlock on error in mpt_config()

We introduced a new return here and forgot to unlock the mutex.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: "Nandigama, Nagalakshmi" <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>