libertas: replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show
functions:
WARNING  use scnprintf or sprintf

Use sysfs_emit instead of scnprintf or sprintf makes more sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Ye Guojin <ye.guojin@zte.com.cn>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211022090438.1065286-1-ye.guojin@zte.com.cn