platform/x86: think-lmi: Clean up display of current_value on Thinkstation

On ThinkStations on retrieving the attribute value the BIOS appends the
possible values to the string.
Clean up the display in the current_value_show function so the options
part is not displayed.

Fixes: a40cd7ef

 ("platform/x86: think-lmi: Add WMI interface support on Lenovo platforms")
Reported by Mario Limoncello <Mario.Limonciello@amd.com>
Link: https://github.com/fwupd/fwupd/issues/5077#issuecomment-1488730526
Signed-off-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Link: https://lore.kernel.org/r/20230403013120.2105-2-mpearson-lenovo@squebb.ca
Tested-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>