spi: Reorder fields in 'struct spi_message'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size from 112 to 96 bytes.

This should have no real impact on memory allocation because 'struct
spi_message' is mostly used on stack, but it can save a few cycles
when the structure is initialized with spi_message_init() and co.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/c112aad16eb47808e1ec10abd87b3d273c969a68.1677704283.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Mark Brown <broonie@kernel.org>