spi: Remove note about transfer limit for spi_write_then_read()

spi: Remove note about transfer limit for spi_write_then_read()

Originally spi_write_then_read() used a fixed statically allocated
buffer which limited the maximum message size it could handle.  This
restriction was removed a while ago so that we could dynamically
allocate a buffer if required but the kerneldoc was not updated to
reflect this, do so.

Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20200525133120.57273-1-broonie@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>