drivers: spi: stm32: factor out completion routine

Polled and IRQ-driven SPI I/O share code for cleanup and completion,
which can now be factored into its own routine.

This keeps a single point of truth for common paths, which will allow
a subsequent bug fix to happen in one place, and help avoid future
regressions.

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