Bluetooth: serdev: Constify serdev_device_ops

serdev_device_ops is not modified and can be const. Also, remove the
unneeded declaration of it.

Output from the file command before and after:

Before:
   text    data     bss     dec     hex filename
   7192    2408     192    9792    2640 drivers/bluetooth/hci_serdev.o

After:
   text    data     bss     dec     hex filename
   7256    2344     192    9792    2640 drivers/bluetooth/hci_serdev.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>