IB/sysfs: Fix sparse warning on attr_id

Attributed ID was declared as an int while the value should really be big
endian 16.

Fixes: 35c4cbb1 ("IB/core: Create get_perf_mad function in sysfs.c")

Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Reviewed-by: Hal Rosenstock <hal@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>