devlink: Add support for driverinit get value for devlink_port

Add support for "driverinit" configuration mode value for devlink_port
configuration parameters. Add devlink_port_param_driverinit_value_get()
function to help the driver get the value from devlink_port.

Also, move the common code to __devlink_param_driverinit_value_get()
to be used by both device and port params.

v7->v8:
-Add the missing devlink_port_param_driverinit_value_get() declaration.
-Also, order devlink_port_param_driverinit_value_get() after
devlink_param_driverinit_value_get/set() calls

Cc: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>