exynos: spi: Fix calculation of SPI transaction start time

The SPI transaction delay is supposed to be measured from the end of one
transaction to the start of the next. The code does not work that way, so
fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>