spi: fsl_espi: fix din offset

spi: fsl_espi: fix din offset

In the case of SPI_XFER_BEGIN | SPI_XFER_END, the function creates a
buffer of double the size of the transaction, so that it can write the
data in into the second half. It sets the rx_offset to len, and in the
while loop we are setting an internal "din" to buffer + rx_offset.

However, at the end of each loop, the driver copies "buffer + 2 *
cmd_len" back to the data_in pointer.

This commit changes the source of the data to buffer + rx_offset.

Signed-off-by: Tomas Alvarez Vanoli <tomas.alvarez-vanoli@hitachienergy.com>