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