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

drivers/spi/spi.c

@@ -3858,8 +3858,7 @@ static u8	*buf;
  * is zero for success, else a negative errno status code.
  * This call may only be used from a context that may sleep.
  *
- * Parameters to this routine are always copied using a small buffer;
- * portable code should never use this for more than 32 bytes.
+ * Parameters to this routine are always copied using a small buffer.
  * Performance-sensitive or bulk transfer code should instead use
  * spi_{async,sync}() calls with dma-safe buffers.
  *