mac802154: ops: declare channel and page as u8

The range of channel and page fits into an unsigned byte range. This
patch changes the set_channel parameter definitions for channel and
page to u8.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Cc: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>