spi: dw: Rearrange struct dw_spi_priv

This should reduce the size of the struct, and also groups more similar
fields together.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Tested-by Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>