spi: Order fields in spi_device for better packing

Now that spi_device->mode is a u16, the chip_select, bits_per_mode,
and mode fields pack poorly, taking 8 bytes:  four data and four
padding.  By moving (u8)bits_per_word up one position, to after
(u8)chip_select, they pack better and only use 4 bytes.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>
Reviewed-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com&gt;g>
Signed-off-by: Mark Brown <broonie@linaro.org>