Bluetooth: hci: spi: abstract out legacy SPI API

Some targets (e.g. STM32-based boards) only have SPI drivers using the
new API. To support both these and existing, legacy SPI drivers in the
SPI HCI driver, abstract out the SPI API into shim routines.

There are no behavioral differences due to this patch. The next patch
will add support for the new API.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>