spi: expose master transfer size limitation.

On some SPI controllers it is not feasible to transfer arbitrary amount
of data at once.

When the limit on transfer size is a few kilobytes at least it makes
sense to use the SPI hardware rather than reverting to gpio driver.

The protocol drivers need a way to check that they do not sent overly
long messages, though.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>