hamradio: baycom: make hdlcdrv_ops const

Make hdlcdrv_ops structures const as they are only passed to
hdlcdrv_register function. The corresponding argument is of type const,
so make the structures const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>