staging: fbtft: convert sysfs snprintf to sysfs_emit

Fix the following coccicheck warning:
drivers/staging/fbtft//fbtft-sysfs.c:202:8-16:
WARNING: use scnprintf or sprintf

Signed-off-by: Xuezhi Zhang <zhangxuezhi1@yulong.com>
Link: https://lore.kernel.org/r/20210403140722.169623-1-llyz108@163.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>