s390/qeth: Make hw_trap sysfs attribute idempotent

s390/qeth: Make hw_trap sysfs attribute idempotent

Update qeth driver to allow writing an existing value to the "hw_trap"
sysfs attribute. Attempting such a write earlier resulted in -EINVAL.
In other words, make the sysfs attribute idempotent.

After:
    $ cat hw_trap
    disarm
    $ echo disarm > hw_trap
    $

Suggested-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Aswin Karuvally <aswin@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250718141711.1141049-1-wintera@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>