spi: add spi_message_init_no_memset to avoid zeroing the spi_message

In the spi_loopback_test driver there is the need to initialize
a spi_message that is filled with values from a static structure.

Applying spi_message_init to such a prefilled structure results in
all the settings getting reset to zero, which is not what we want.
Copying each field of spi_message separately instead always includes
the risk that some new fields have not been implemented in the copying
code.

So here we introduce a version of spi_message_init called
spi_message_init_no_memset that does not fill the structure
with zero first, but only initializes the relevant list_heads.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>