s390/qeth: Convert sysfs sprintf to sysfs_emit

Following the advice of the Documentation/filesystems/sysfs.rst.
All sysfs related show()-functions should only use sysfs_emit() or
sysfs_emit_at() when formatting the value to be returned to user space.

Reported-by: Jules Irenge <jbi.octave@gmail.com>
Reported-by: Joe Perches <joe@perches.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>