staging: fieldbus: use sysfs_emit() in show functions

The show() methods should only use sysfs_emit() when formatting values
to be returned to the user space.
Ref: Documentation/filesystems/sysfs.rst
Issue identified by coccicheck.

Signed-off-by: Deepak R Varma <drv@mailo.com>
Link: https://lore.kernel.org/r/Y2uEIzebbM/Fs5Jz@qemulion


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