s390/qeth: return error when starting a reset fails

When starting the reset worker via sysfs is unsuccessful, return an
error to the user.
Modernize the sysfs input parsing while at it.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>