spi: replace snprintf in show functions with sysfs_emit

show() must not use snprintf() when formatting the value to be
returned to user space.

Fix the following coccicheck warning:
drivers/spi/spi-tle62x0.c:144: WARNING: use scnprintf or sprintf.

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Link: https://lore.kernel.org/r/1634280668-4954-1-git-send-email-wangqing@vivo.com
Signed-off-by: Mark Brown <broonie@kernel.org>