caif-hsi: Add rtnl support

Add RTNL support for managing the caif hsi interface.
The HSI HW interface is no longer registering as a device,
instead we use symbol_get to get hold of the HSI API.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>