spi/trace: Cap buffer contents at 64 bytes

Large transfers (64kB) doesn't show up in the trace. Not sure why, but
since printk can only display buffers up to 64 bytes in length, we only
need to store the first 64 bytes.

Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Mark Brown <broonie@kernel.org>