spi: core: Add spi_transfer_is_last() helper

This adds the function spi_transfer_is_last() which can be used by
drivers to know whether a given transfer is the last one in the
current message.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>