IB/hfi1: Clean up type used and casting

In all other places in this file where 'find_first_bit' is called,
port_num is defined as a 'u8' and no casting is done.

Do the same here in order to be more consistent.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Doug Ledford <dledford@redhat.com>