spi/s3c64xx: Log error interrupts

Although the hardware supports interrupts we're not currently using them
at all since for small transfers the overhead is greater than that for
busy waiting and for large transfers we have interrupts from the DMA.
This means that if the hardware reports an error (especially one which
might not stall transfer) we might miss it.

Take a first pass at dealing with such errors by enabling the interrupt
if we can and logging the errors if they happen. Ideally we'd report the
error via the affected transfer but since we're in master mode it's very
difficult to trigger errors at present and this code is much simpler.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>