caif-hsi: HSI-Platform device register and unregisters itself

Platform device is no longer removed from caif_hsi at shutdown.
The HSI-platform device must do it's own registration and unregistration.

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