spi: fsl-espi: factor out filling the local buffer

Better structure the code by factoring out filling the local buffer.

In addition don't initialize the complete local buffer at the
beginning of fsl_espi_do_one_msg. Instead move initialization of
those parts of the local buffer to be used for transfers w/o tx_buf
to fsl_espi_copy_to_buf.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>