spi/s3c64xx: Split wait_for_xfer() into PIO and DMA versions

There is no meaningful code sharing between the PIO and DMA variants
(just the timeout calculation) so in order to make the code easier to
work with split the two cases.

Looking at the code it is not clear how the PIO version works for large
transmits, greater than FIFO size is only handled for RX.

Signed-off-by: Mark Brown <broonie@linaro.org>