IB/ipath: Calling PTR_ERR() on right variable in create_file()

"dentry" is a valid pointer.  "*dentry" was intended.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>