sdio: don't use CAM_PRIORITY_NONE for queued CCB-s

This is similar to changes done in other CAM drivers and fixes a panic
because of a sanity check added in b4b166b8c46b.

While here, remove unneeded ccb setup in sdiobdiscover.
It's possible that ccb allocation in that function is not needed as
well, but I wasn't sure about that.

MFC after: 1 week