platform/x86: msi-wmi-platform: Fix typo in WMI GUID

platform/x86: msi-wmi-platform: Fix typo in WMI GUID

The WMI driver core only supports GUID strings containing only
uppercase characters, however the GUID string used by the
msi-wmi-platform driver contains a single lowercase character.
This prevents the WMI driver core from matching said driver to
its WMI device.

Fix this by turning the lowercase character into a uppercase
character. Also update the WMI driver development guide to warn
about this.

Reported-by: Antheas Kapenekakis <lkml@antheas.dev>
Fixes: 9c0beb6b29e7 ("platform/x86: wmi: Add MSI WMI Platform driver")
Tested-by: Antheas Kapenekakis <lkml@antheas.dev>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20251110111253.16204-3-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>