platform/x86: dell-wmi-sysman: Use sysfs_emit{_at} in show functions

platform/x86: dell-wmi-sysman: Use sysfs_emit{_at} in show functions

Replace sprintf() with sysfs_emit() and sysfs_emit_at() in sysfs show
functions. sysfs_emit() and sysfs_emit_at() are preferred for formatting
sysfs output because they provide safer bounds checking.

In reset_bios_show(), use sysfs_emit_at() to avoid manual buffer size
accounting.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260305140912.258090-2-thorsten.blum@linux.dev
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>