[SCSI] mvsas: fix potential NULL dereference

Stanse found that in mvs_abort_task, mvi_dev is dereferenced earlier
than tested for being NULL. Move the assignment below the test.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>