staging: most: hdm-usb: unify returned errors

This patch is needed to return the same error codes for the same error
cases in the functions show_value() and store_value().

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>