drivers: spi: stm32: add error checking to IRQ mode

Now that struct spi_context supports passing errors from
interrupt-driven I/O handlers to waiting threads, we can enable error
interrupts and propagate errors to spi_transceive() callers.

To make it easier for users to debug SPI-related issues, log any error
bits set in SR when failures occur.

A subsequent patch will add error checking to polled mode as well, but
other cleanups and fixes will go in first to make this easier.

Note that this breaks the spi_loopback test on some targets, but it's
not a regression, as it wasn't working properly anyway. Subsequent
patches the bugs that this error checking has exposed.

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