drivers: spi: stm32: fix SPI being left on in slave mode

The current implementation unconditionally enables the SPI (sets
SPI_CR1_SPE) in transceive(), but disables it only in master mode.

The peripheral should only be enabled while the user has specifically
requested I/O. Fix this by always disabling the peripheral when I/O is
complete.

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