drivers: spi: stm32: fix bugs in polled mode

In polled mode, the STM32 SPI driver is signaling completion when
there are no waiters:

- the only spi_context_wait_for_completion() caller in this driver is
  in the IRQ-driven portion of transceive() itself, which isn't
  compiled in polled mode.

- the "asynchronous completion + polled I/O" combination is not
  supported by the driver, so there are no other threads polling on
  this I/O we need to signal completion to.

What should be happening instead of signaling completion is releasing
the chip select pin, which polled I/O currently doesn't do.

Fix these issues.

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