drivers: spi hci: Add spi dedicated rx thread

Get spi hci driver to define its own rx thread, in order
not to mix with bt own RX thread as driver use bt_recv_prio
that expect to be used in a different thread than BT host one.

Fixes #15714

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>