platform/x86: dell-smbios-wmi: release mutex lock on WMI call failure

Unbound devices may race with calling this function causing the mutex
to stay locked.  This failure mode should have released the mutex too.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>