s390/qeth: convert sysfs snprintf to sysfs_emit

Follow the advice of the Documentation/filesystems/sysfs.rst
and show() should only use sysfs_emit() or sysfs_emit_at()
when formatting the value to be returned to user space.

Signed-off-by: Xuezhi Zhang <zhangxuezhi1@coolpad.com>
Signed-off-by: David S. Miller <davem@davemloft.net>