spi: mpc8xxx: Re-order transfer setup

Minize the time the adapter is disabled (via SPI_MODE_EN
clearing/setting) to just the character length setting, and only set up
the temporary data writing variable right before we need it, so there is
a more clear distinction between setting up the SPI adapter, and setting
up the data to be written.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>