net: ethernet: qualcomm: constify qca_serdev_ops

The only usage of qca_serdev_ops is to pass its address to
serdev_device_set_client_ops() which takes a const pointer. Make it
const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>