drivers: bluetooth: hci_ambiq: add necessary SPI CS control

Before sending packet to controller the host needs to poll the status of
controller to know it's ready, or before reading packets from controller
the host needs to get the payload size of coming packets by sending
specific command and putting the status or size to the rx buffer, the CS
should be held at this moment to continue to send or receive packets.
This change is needed for the based SPI driver update.

Signed-off-by: Aaron Ye <aye@ambiq.com>