caif-hsi: Postpone init of HSI until open()

Do the initialization of the HSI interface when the
interface is opened, instead of upon registration.
When the interface is closed the HSI interface is
de-initialized, allowing other modules to use the
HSI interface.

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